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Overgrowth of GaN on Be-doped coalesced GaN nanocolumn layer by rf-plasma-assisted molecular-beam epitaxy—Formation of high-quality GaN microcolumns
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2956–2961
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Overgrowth of GaN on Be-doped coalesced GaN nanocolumn layer by rf-plasma-assisted molecular-beam epitaxy—Formation of high-quality GaN microcolumns Kei Kato a, Katsumi Kishino a,b,c,, Hiroto Sekiguchi a,c, Akihiko Kikuchi a,b,c a
Department of Engineering and Applied Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan Sophia Nanotechnology Research Center, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan c CREST, Japan Science and Technology Agency, Kawaguchi, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 19 January 2009
GaN crystals were overgrown on GaN nanocolumn platforms with a Be-doped coalesced layer by rfplasma-assisted molecular-beam epitaxy (rf-MBE). The overgrown GaN included large micrometerscale hexagonal columnar crystals. These microcrystals were named ‘microcolumns’ and showed high optical properties comparable to those of GaN bulk crystals grown by hydride vapor phase epitaxy (HVPE). & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.Et 78.67. n 61.46.Hk 81.15.Hi Keywords: A1. Nanostructures A3. Molecular-beam epitaxy B1. Microcolumn B1. Nanocolumn B1. Nitrides B2. Semiconducting III–V materials
1. Introduction GaN substrates with fewer threading dislocations are necessary for fabricating high-performance III-nitride devices. Commercialized InGaN-based light-emitting diodes (LEDs) are fabricated on c-axis sapphire substrates using low-temperaturegrown GaN buffer layers. However, in InGaN/GaN semiconductor lasers, GaN substrates are essential for improving their reliability. Most GaN substrates are fabricated by removing seed substrates after growing thick GaN films on them; for example, GaAs substrates are removed after growing thick GaN films on them by hydride vapor-phase epitaxy (HVPE) [1]. However, the cost of GaN substrates is still very high. Recently, InGaN emitters on semipolar (11 2¯ 2) and non-polar m-plane (11¯ 0 0) GaN substrates have been studied for the purpose of developing highly efficient green LEDs and green laser diodes [2–4]. At the same time, GaN nanocolumns are known as one of the high-quality crystals. They are one-dimensional columnar nanocrystals with 20–200 nm
Corresponding author at: Department of Engineering and Applied Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan. Tel.: +81 3 3238 3323; fax: +81 3 3238 3321. E-mail address: [email protected] (K. Kishino).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.057
diameter, 0.5–2 mm height and a 1010 cm 2 density [5] and have high optical properties due to their dislocation-free nature [6]. We reported the epitaxial overgrowth of a continuous GaN film on GaN nanocolumns in 2002 [7], and then the enhancement effect of Mg doping on coalesced GaN nanocolumns was utilized for the fabrication of InGaN/GaN nanocolumn LEDs [8]. Very recently, we have reported that Be doping had a superior enhancement effect on the lateral growth of GaN nanocolumns [9]. GaN overgrowth on GaN nanocolumns can provide a new fabrication technique for GaN substrates because of the easy removal of the substrates. In this paper, we investigated GaN crystals overgrown on a Be-doped coalesced GaN nanocolumn layer prepared on (111) Si substrates by rf-plasma-assisted molecular-beam epitaxy (rf-MBE).
2. Experiments GaN crystals were overgrown on nanocolumn platforms by rfMBE as shown in Fig. 1. The platforms were prepared by growing GaN nanocolumns on n-type (111) Si substrates at 940 1C for 40 min followed by Be-doped GaN coalesced layers at 770 1C for 90 min. As reported in Ref. [9], Be doping into the GaN nanocolumns enhanced the lateral growth of nanocolumns to
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produce a smooth and continuous film on the top surface; the root-mean-square (RMS) surface roughness obtained by atomic force microscope (AFM) observation was markedly reduced from 35 to 10 nm over an area of 20 mm 20 mm upon increasing the Be cell temperature (TBe) from 710 to 975 1C. From cross-sectional and plan-view transmission electron microscope (TEM) observations of the Be-doped nanocolumn grown at TBe=920 1C, it was observed that the nanocolumn size increased in the Bedoped region due to lateral growth, but the coalescence between
n-GaN:Si
Overgrowth layer
GaN:Be Nanocolumn platform n-GaN:Si
n(111)Si Fig. 3. Plan-view bright-field TEM image of overgrowth GaN (sample B). Fig. 1. Schematic diagram of overgrowth GaN crystals.
Fig. 2. Bird’s-eye SEM images: (a) overgrowth layer of sample A and (b) overgrowth layer of sample B, (c) Be-doped nanocolumn platform (Tbe=920 1C) for sample A and (d) Be-doped nanocolumn platform (Tbe=975 1C) for sample B.
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nanocolumns was insufficient. However, no threading dislocations were observed in the single enlarged nanocolumn. The Be-doped GaN coalesced layers (TBe=920 and 975 1C) were utilized as nanocolumn platforms for the overgrowth. Bird’s-eye views of these nanocolumn platforms are shown in Fig. 2(c) and (d), respectively; these nanocolumn platforms included a 5-nmthick InGaN single quantum well between the Be-doped GaN coalesced layer and the Si-doped GaN nanocolumns, which was different from the nanocolumn platforms used for the overgrowth. The growth duration of the n-type GaN nanocolumns was 60 min, which was long compared with that of the overgrowth samples. However, the presence of the 5-nm-thick InGaN layer and the short growth time of the GaN nanocolumns do not change the surface morphology. The RMS surface roughnesses with the nanocolumn platforms of TBe=920 and 975 1C were 22.5 and 10.4 nm, respectively [9]. In this investigation, the Ga beam equivalent pressure (Ga-BEP), growth time and Be cell temperature were varied as growth parameters, where the III element (Ga) and dopants (Si and Be) were supplied from standard effusion cells and active nitrogen was supplied from an rf-plasma nitrogen source. Three Si-doped GaN overgrown samples (samples A, B and C) were
Fig. 4. SEM images of GaN microcolumns: (a) Bird’s-eye-view image and (b) crosssectional image.
fabricated on the nanocolumn platforms; the Be cell temperature of the Be-doped coalesced layer was 920 1C for samples A and C and 975 1C for sample B. Samples A and B were grown under the same Ga-BEP of 1.0 10 4 Pa at 770 1C, but with the different growth durations of 240 and 270 min, respectively. Sample C was grown at 780 1C for 150 min with a higher Ga-BEP of 5.0 10 4 Pa. The nitrogen flow rate was 3 sccm for all samples. The prepared samples were evaluated by scanning electron microscope (SEM), AFM, TEM and scanning transmission electron microscope (STEM), and observing cathode luminescence (CL) and photoluminescence (PL).
3. Results and discussion 3.1. SEM observation Fig. 2(a)–(d) shows bird’s-eye view SEM images of samples A and B and the GaN nanocolumn platforms of samples A and B grown at TBe=920 and 975 1C, respectively. The height of the GaN nanocolumn platforms was 1.2 mm for both overgrowth samples A and B, and the thickness of the GaN overgrown crystal was 0.5 mm for sample A and 1.5 mm for sample B. Thus, the growth rates of samples A and B were 0.1 and 0.3 mm/h, respectively. When TBe=920 1C, the coalescence in the Be-doped GaN layer was insufficient as described above. Therefore, the growth of GaN inside the nanocolumns probably proceeded through the fissures that opened on the surface and the growth rate of sample A was thus lower. Many hexagonal grain structures with a diameter of less than 1.2 mm and a density of 1.2 108 cm 2 were observed on the surface of sample A. On the other hand, for sample B, the grain diameter increased up to 2 mm and the grain density decreased by two orders of magnitude to 1.5 106 cm 2; thus, the surface smoothness was improved. On the nanocolumn platform surfaces corresponding to samples A and B, smaller hexagonal grains of diameter less than 0.7 mm appeared with densities of 5.6 107 and 1.4 106 cm 2, respectively (see Fig. 2(c) and (d)). It is interesting to note that the grain diameter increased for both samples with the overgrowth, indicating lateral growth; meanwhile, the grain density increased for sample A after the overgrowth, but it remained almost constant for sample B. The crystal orientation of the hexagonal grains was preserved after the overgrowth. In fact, the hexagonal grains on the surfaces of samples A and B and the Be-doped GaN nanocolumn platform of
Fig. 5. Top-view SEM image (a) and AFM image (b) of Be-doped GaN nanocolumn platform for sample A.
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Fig. 6. GaN microcolumns: (a) top SEM image and (b) CL image.
sample A were aligned in the same crystal direction; two sides of the hexagonal grains were parallel to the [11 0] direction of the Si substrates. For the grains on the nanocolumn platform of sample B, the crystal direction could not be determined. The experiment proved that the surface morphology of the overgrowth crystals is determined by the surface roughness of the underlying nanocolumn platform. The RMS surface roughness of sample B evaluated by AFM over an area of 20 mm 20 mm was 35 nm. However, unfortunately many cracks were observed on the surfaces of both samples A and B. In the plan-view TEM image of sample B, many dislocations were observed on the overgrowth surface corresponding to the coalescence boundaries between nanocolumns as shown in Fig. 3, in which the contrast resulting from dislocations in the form of curved lines or circles appeared. The PL peak intensity from sample B was thus very weak and only 3% of that of a metal-organic vapor-phase epitaxy (MOVPE)-grown Si-doped GaN template (7 mm), which had a threading dislocation density (TDD) and a carrier density of 6.0 108 cm 2 and 1–3 1018 cm 3, respectively. Thus, the overgrowth of GaN on self-assembled nanocolumns is not a suitable method for the fabrication of high-quality GaN substrates. The random and spontaneous nucleation of nanocolumns in the self-assembly process introduces randomness in the position and size of nanocolumns, which brings about fluctuations in the coalescence. The periodic arrangement of nanocolumns with the same size and the careful control of coalescence will contribute to the successful fabrication of high-quality GaN substrates with a low dislocation density. Fig. 4(a) shows a bird’s-eye-view SEM image of sample C; many large micrometer-scale hexagonal columnar crystals were observed, which were named as ‘microcolumns’. The designed height of the nanocolumn platform was 1.2 mm and the total
Fig. 7. PL spectra of GaN microcolumn (sample C) and HVPE-grown GaN substrate.
thickness of this sample was 5.2 mm; thus, the height of the microcolumns is estimated to be 4 mm. The top facets of the microcolumns appear to be atomically smooth, and the AFM observation over an area of 0.5 mm 0.5 mm gave an RMS surface roughness of as low as 0.8 nm. The diameter of the microcolumns increased from 0.7 (at the bottom) to 2 mm (at the top) during the growth and the surface density was approximately 8.7 106 cm 2. The crystal orientation of the side facets was the same for all microcolumns, suggesting that the microcolumns grew with the crystal orientation of the seed crystals on the underlying nanocolumn platform. Observing the bottom of the nanocolumn platform in Fig. 4(b), it can be seen that most of the space between the Si-doped nanocolumns was filled with GaN, similarly to that in sample A.
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Fig. 8. Plan-view bright-field STEM image (a) and bright-field TEM image and (b) of GaN microcolumn (sample C).
It is speculated that the hexagonal grains on the nanocolumn platform function as seeds for the growth of microcolumns. Figs. 5(a) and (b), respectively, show the SEM and AFM images of the Be-doped nanocolumn platform of TBe=920 1C (also see Fig. 2(c)), on which hexagonal grains with a diameter of approximately 0.5 mm were observed; this value was close to the diameter of the microcolumns at the bottom, 0.7 mm. The grain density was 5.6 107 cm 2, which was comparable to the microcolumn density of 8.7 106 cm 2. Thus, the microcolumn growth probably initiated from some of the grain structures on the surface of the Be-doped GaN nanocolumn platform. Figs. 5(a) and (b) show that the GaN microcolumns tend to grow independently of other microcolumns. The microcolumns appeared only for a higher Ga-BEP of 5.0 10 4 Pa, but lateral growth occurred for a low Ga-BEP of 1.0 10 4 Pa (see Fig. 2). Thus, the vertical growth rate along the direction of the [0 0 0 1] crystal axis increased relative to the horizontal growth rate with increase in Ga-BEP. Two sides of the hexagonal face of all microcolumns were perpendicular to the [11 0] axis of the Si substrate, which is different from the case of the overgrown samples A and B. The reason for this difference is still not known. The microcolumns have the potential to provide high-quality crystals for the fabrication of micrometer-size devices. In the following sections, the crystal qualities of the microcolumns are discussed.
strain in the microcolumns. The PL peak intensity of sample C was one-third as strong as that of the GaN substrate. The CL observation proved that the PL from sample C is mostly radiated from the microcolumns. From the microcolumn diameter of 2 mm and the density of 8.7 106 cm 2, the surface occupation coefficient of the microcolumns is calculated to be approximately 0.22; this indicates that the PL intensity of the microcolumns is comparable to that of HVPE-grown GaN. 3.3. TEM observation of microcolumns The microcolumns were evaluated by STEM and TEM observation. Plan-view bright-field STEM and bright-field TEM images of the top surface of a microcolumn are shown in Figs. 8(a) and (b), respectively. No dislocations were observed in the microcolumn, although many contrast lines related to interference appeared in the images; the contrasts were introduced as a result of the bending of the thin TEM sample. Not only bright-field STEM and TEM images but also dark-field TEM images were obtained, and no dislocations were observed in the microcolumn. However, in some other microcolumns, threading dislocations were observed. However, as the microcolumns possess the strong PL intensity comparable to that of HVPE-grown GaN, relatively few microcolumns can be considered to contain dislocations.
4. Conclusion 3.2. Emission properties of microcolumns Figs. 6(a) and (b) show an SEM top image and the corresponding CL image at the same area of the microcolumns (in sample C), respectively. The CL measurement was performed under electron-beam irradiation with an acceleration voltage of 5 kV. Three microcolumns of 2 mm diameter were observed in the SEM image; the CL from the corresponding three microcolumns was strong when compared with that from the area outside the microcolumns. The room-temperature PL property of the overgrowth sample with the microcolumns (sample C) was evaluated under He–Cd laser (325 nm) light irradiation. For comparison, HVPE-grown undoped GaN bulk crystal with a TDD of 6.0 106 cm 2 was also evaluated. Fig. 7 shows the PL spectra of a GaN microcolumn (from sample C) and the HVPE-grown GaN substrate. The peak wavelength of sample C was 364.7 nm, longer than that of the GaN substrate (363.8 nm), indicating the presence of residual tensile
GaN crystals were overgrown on Be-doped GaN nanocolumn platforms, which were prepared on an n-type (111) Si substrate. GaN overgrowth crystals grown with a low Ga-BEP of 1.0 10 4 Pa produce continuous films but with a high dislocation density and weak PL emission. Thus, the overgrowth of GaN on self-assembled nanocolumns is not a suitable method for the fabrication of GaN substrates with high crystal quality. In the overgrowth crystals grown under a higher Ga-BEP of 5.0 10 4 Pa, many micrometerscale hexagonal columnar crystals, named ‘microcolumns’, were observed. CL and PL measurements demonstrated the high optical properties of the microcolumns.
Acknowledgements This study is partly supported by a Grant-in-Aid for Scientific Research on Priority Areas #17656026 and (B) #18310079
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from the Ministry of Education, Culture, Sports, Science and Technology. References [1] K. Motoki, et al., Jpn. J. Appl. Phys. 40 (2001) L140. [2] M. Funato, M. Ueda, Y. Kawakami, Y. Narukawa, T. Kosugi, M. Takahashi, T. Mukai, Jpn. J. Appl. Phys. 45 (2006) L659. [3] M.C. Schmidt, K. Kim, R.M. Farrell, D.F. Feezell, D.A. Cohen, M. Saito, K. Fujito, J.S. Speck, S.P. DenBaars, S. Nakamura, Jpn. J. Appl. Phys. 46 (2007) L190.
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[4] K. Okamoto, H. Ohta, S.F. Chchibu, J. Ichihara, H. Takasu, Jpn. J. Appl. Phys. 46 (2007) L187. [5] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, K. Kishino, Jpn. J. Appl. Phys. 36 (1997) L459. [6] H. Sekiguchi, T. Nakazato, A. Kikuchi, K. Kishino, J. Crystal Growth 300 (2007) 259. [7] K. Kusakabe, A. Kikuchi, K. Kishino, J. Crystal Growth 237–239 (2002) 988. [8] A. Kikuchi, M. Kawai, M. Tada, K. Kishino, Jpn. J. Appl. Phys. 43 (2004) L1524. [9] H. Sekiguchi, K. Kato, A. Kikuchi, K. Kishino, Phys. Status Solidi C 5 (2008) 3069.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Overgrowth of GaN on Be-doped coalesced GaN nanocolumn layer by rf-plasma-assisted molecular-beam epitaxy—Formation of high-quality GaN microcolumns Kei Kato a, Katsumi Kishino a,b,c,, Hiroto Sekiguchi a,c, Akihiko Kikuchi a,b,c a
Department of Engineering and Applied Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan Sophia Nanotechnology Research Center, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan c CREST, Japan Science and Technology Agency, Kawaguchi, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 19 January 2009
GaN crystals were overgrown on GaN nanocolumn platforms with a Be-doped coalesced layer by rfplasma-assisted molecular-beam epitaxy (rf-MBE). The overgrown GaN included large micrometerscale hexagonal columnar crystals. These microcrystals were named ‘microcolumns’ and showed high optical properties comparable to those of GaN bulk crystals grown by hydride vapor phase epitaxy (HVPE). & 2009 Elsevier B.V. All rights reserved.
PACS: 78.55.Et 78.67. n 61.46.Hk 81.15.Hi Keywords: A1. Nanostructures A3. Molecular-beam epitaxy B1. Microcolumn B1. Nanocolumn B1. Nitrides B2. Semiconducting III–V materials
1. Introduction GaN substrates with fewer threading dislocations are necessary for fabricating high-performance III-nitride devices. Commercialized InGaN-based light-emitting diodes (LEDs) are fabricated on c-axis sapphire substrates using low-temperaturegrown GaN buffer layers. However, in InGaN/GaN semiconductor lasers, GaN substrates are essential for improving their reliability. Most GaN substrates are fabricated by removing seed substrates after growing thick GaN films on them; for example, GaAs substrates are removed after growing thick GaN films on them by hydride vapor-phase epitaxy (HVPE) [1]. However, the cost of GaN substrates is still very high. Recently, InGaN emitters on semipolar (11 2¯ 2) and non-polar m-plane (11¯ 0 0) GaN substrates have been studied for the purpose of developing highly efficient green LEDs and green laser diodes [2–4]. At the same time, GaN nanocolumns are known as one of the high-quality crystals. They are one-dimensional columnar nanocrystals with 20–200 nm
Corresponding author at: Department of Engineering and Applied Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan. Tel.: +81 3 3238 3323; fax: +81 3 3238 3321. E-mail address: [email protected] (K. Kishino).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.057
diameter, 0.5–2 mm height and a 1010 cm 2 density [5] and have high optical properties due to their dislocation-free nature [6]. We reported the epitaxial overgrowth of a continuous GaN film on GaN nanocolumns in 2002 [7], and then the enhancement effect of Mg doping on coalesced GaN nanocolumns was utilized for the fabrication of InGaN/GaN nanocolumn LEDs [8]. Very recently, we have reported that Be doping had a superior enhancement effect on the lateral growth of GaN nanocolumns [9]. GaN overgrowth on GaN nanocolumns can provide a new fabrication technique for GaN substrates because of the easy removal of the substrates. In this paper, we investigated GaN crystals overgrown on a Be-doped coalesced GaN nanocolumn layer prepared on (111) Si substrates by rf-plasma-assisted molecular-beam epitaxy (rf-MBE).
2. Experiments GaN crystals were overgrown on nanocolumn platforms by rfMBE as shown in Fig. 1. The platforms were prepared by growing GaN nanocolumns on n-type (111) Si substrates at 940 1C for 40 min followed by Be-doped GaN coalesced layers at 770 1C for 90 min. As reported in Ref. [9], Be doping into the GaN nanocolumns enhanced the lateral growth of nanocolumns to
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produce a smooth and continuous film on the top surface; the root-mean-square (RMS) surface roughness obtained by atomic force microscope (AFM) observation was markedly reduced from 35 to 10 nm over an area of 20 mm 20 mm upon increasing the Be cell temperature (TBe) from 710 to 975 1C. From cross-sectional and plan-view transmission electron microscope (TEM) observations of the Be-doped nanocolumn grown at TBe=920 1C, it was observed that the nanocolumn size increased in the Bedoped region due to lateral growth, but the coalescence between
n-GaN:Si
Overgrowth layer
GaN:Be Nanocolumn platform n-GaN:Si
n(111)Si Fig. 3. Plan-view bright-field TEM image of overgrowth GaN (sample B). Fig. 1. Schematic diagram of overgrowth GaN crystals.
Fig. 2. Bird’s-eye SEM images: (a) overgrowth layer of sample A and (b) overgrowth layer of sample B, (c) Be-doped nanocolumn platform (Tbe=920 1C) for sample A and (d) Be-doped nanocolumn platform (Tbe=975 1C) for sample B.
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nanocolumns was insufficient. However, no threading dislocations were observed in the single enlarged nanocolumn. The Be-doped GaN coalesced layers (TBe=920 and 975 1C) were utilized as nanocolumn platforms for the overgrowth. Bird’s-eye views of these nanocolumn platforms are shown in Fig. 2(c) and (d), respectively; these nanocolumn platforms included a 5-nmthick InGaN single quantum well between the Be-doped GaN coalesced layer and the Si-doped GaN nanocolumns, which was different from the nanocolumn platforms used for the overgrowth. The growth duration of the n-type GaN nanocolumns was 60 min, which was long compared with that of the overgrowth samples. However, the presence of the 5-nm-thick InGaN layer and the short growth time of the GaN nanocolumns do not change the surface morphology. The RMS surface roughnesses with the nanocolumn platforms of TBe=920 and 975 1C were 22.5 and 10.4 nm, respectively [9]. In this investigation, the Ga beam equivalent pressure (Ga-BEP), growth time and Be cell temperature were varied as growth parameters, where the III element (Ga) and dopants (Si and Be) were supplied from standard effusion cells and active nitrogen was supplied from an rf-plasma nitrogen source. Three Si-doped GaN overgrown samples (samples A, B and C) were
Fig. 4. SEM images of GaN microcolumns: (a) Bird’s-eye-view image and (b) crosssectional image.
fabricated on the nanocolumn platforms; the Be cell temperature of the Be-doped coalesced layer was 920 1C for samples A and C and 975 1C for sample B. Samples A and B were grown under the same Ga-BEP of 1.0 10 4 Pa at 770 1C, but with the different growth durations of 240 and 270 min, respectively. Sample C was grown at 780 1C for 150 min with a higher Ga-BEP of 5.0 10 4 Pa. The nitrogen flow rate was 3 sccm for all samples. The prepared samples were evaluated by scanning electron microscope (SEM), AFM, TEM and scanning transmission electron microscope (STEM), and observing cathode luminescence (CL) and photoluminescence (PL).
3. Results and discussion 3.1. SEM observation Fig. 2(a)–(d) shows bird’s-eye view SEM images of samples A and B and the GaN nanocolumn platforms of samples A and B grown at TBe=920 and 975 1C, respectively. The height of the GaN nanocolumn platforms was 1.2 mm for both overgrowth samples A and B, and the thickness of the GaN overgrown crystal was 0.5 mm for sample A and 1.5 mm for sample B. Thus, the growth rates of samples A and B were 0.1 and 0.3 mm/h, respectively. When TBe=920 1C, the coalescence in the Be-doped GaN layer was insufficient as described above. Therefore, the growth of GaN inside the nanocolumns probably proceeded through the fissures that opened on the surface and the growth rate of sample A was thus lower. Many hexagonal grain structures with a diameter of less than 1.2 mm and a density of 1.2 108 cm 2 were observed on the surface of sample A. On the other hand, for sample B, the grain diameter increased up to 2 mm and the grain density decreased by two orders of magnitude to 1.5 106 cm 2; thus, the surface smoothness was improved. On the nanocolumn platform surfaces corresponding to samples A and B, smaller hexagonal grains of diameter less than 0.7 mm appeared with densities of 5.6 107 and 1.4 106 cm 2, respectively (see Fig. 2(c) and (d)). It is interesting to note that the grain diameter increased for both samples with the overgrowth, indicating lateral growth; meanwhile, the grain density increased for sample A after the overgrowth, but it remained almost constant for sample B. The crystal orientation of the hexagonal grains was preserved after the overgrowth. In fact, the hexagonal grains on the surfaces of samples A and B and the Be-doped GaN nanocolumn platform of
Fig. 5. Top-view SEM image (a) and AFM image (b) of Be-doped GaN nanocolumn platform for sample A.
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Fig. 6. GaN microcolumns: (a) top SEM image and (b) CL image.
sample A were aligned in the same crystal direction; two sides of the hexagonal grains were parallel to the [11 0] direction of the Si substrates. For the grains on the nanocolumn platform of sample B, the crystal direction could not be determined. The experiment proved that the surface morphology of the overgrowth crystals is determined by the surface roughness of the underlying nanocolumn platform. The RMS surface roughness of sample B evaluated by AFM over an area of 20 mm 20 mm was 35 nm. However, unfortunately many cracks were observed on the surfaces of both samples A and B. In the plan-view TEM image of sample B, many dislocations were observed on the overgrowth surface corresponding to the coalescence boundaries between nanocolumns as shown in Fig. 3, in which the contrast resulting from dislocations in the form of curved lines or circles appeared. The PL peak intensity from sample B was thus very weak and only 3% of that of a metal-organic vapor-phase epitaxy (MOVPE)-grown Si-doped GaN template (7 mm), which had a threading dislocation density (TDD) and a carrier density of 6.0 108 cm 2 and 1–3 1018 cm 3, respectively. Thus, the overgrowth of GaN on self-assembled nanocolumns is not a suitable method for the fabrication of high-quality GaN substrates. The random and spontaneous nucleation of nanocolumns in the self-assembly process introduces randomness in the position and size of nanocolumns, which brings about fluctuations in the coalescence. The periodic arrangement of nanocolumns with the same size and the careful control of coalescence will contribute to the successful fabrication of high-quality GaN substrates with a low dislocation density. Fig. 4(a) shows a bird’s-eye-view SEM image of sample C; many large micrometer-scale hexagonal columnar crystals were observed, which were named as ‘microcolumns’. The designed height of the nanocolumn platform was 1.2 mm and the total
Fig. 7. PL spectra of GaN microcolumn (sample C) and HVPE-grown GaN substrate.
thickness of this sample was 5.2 mm; thus, the height of the microcolumns is estimated to be 4 mm. The top facets of the microcolumns appear to be atomically smooth, and the AFM observation over an area of 0.5 mm 0.5 mm gave an RMS surface roughness of as low as 0.8 nm. The diameter of the microcolumns increased from 0.7 (at the bottom) to 2 mm (at the top) during the growth and the surface density was approximately 8.7 106 cm 2. The crystal orientation of the side facets was the same for all microcolumns, suggesting that the microcolumns grew with the crystal orientation of the seed crystals on the underlying nanocolumn platform. Observing the bottom of the nanocolumn platform in Fig. 4(b), it can be seen that most of the space between the Si-doped nanocolumns was filled with GaN, similarly to that in sample A.
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Fig. 8. Plan-view bright-field STEM image (a) and bright-field TEM image and (b) of GaN microcolumn (sample C).
It is speculated that the hexagonal grains on the nanocolumn platform function as seeds for the growth of microcolumns. Figs. 5(a) and (b), respectively, show the SEM and AFM images of the Be-doped nanocolumn platform of TBe=920 1C (also see Fig. 2(c)), on which hexagonal grains with a diameter of approximately 0.5 mm were observed; this value was close to the diameter of the microcolumns at the bottom, 0.7 mm. The grain density was 5.6 107 cm 2, which was comparable to the microcolumn density of 8.7 106 cm 2. Thus, the microcolumn growth probably initiated from some of the grain structures on the surface of the Be-doped GaN nanocolumn platform. Figs. 5(a) and (b) show that the GaN microcolumns tend to grow independently of other microcolumns. The microcolumns appeared only for a higher Ga-BEP of 5.0 10 4 Pa, but lateral growth occurred for a low Ga-BEP of 1.0 10 4 Pa (see Fig. 2). Thus, the vertical growth rate along the direction of the [0 0 0 1] crystal axis increased relative to the horizontal growth rate with increase in Ga-BEP. Two sides of the hexagonal face of all microcolumns were perpendicular to the [11 0] axis of the Si substrate, which is different from the case of the overgrown samples A and B. The reason for this difference is still not known. The microcolumns have the potential to provide high-quality crystals for the fabrication of micrometer-size devices. In the following sections, the crystal qualities of the microcolumns are discussed.
strain in the microcolumns. The PL peak intensity of sample C was one-third as strong as that of the GaN substrate. The CL observation proved that the PL from sample C is mostly radiated from the microcolumns. From the microcolumn diameter of 2 mm and the density of 8.7 106 cm 2, the surface occupation coefficient of the microcolumns is calculated to be approximately 0.22; this indicates that the PL intensity of the microcolumns is comparable to that of HVPE-grown GaN. 3.3. TEM observation of microcolumns The microcolumns were evaluated by STEM and TEM observation. Plan-view bright-field STEM and bright-field TEM images of the top surface of a microcolumn are shown in Figs. 8(a) and (b), respectively. No dislocations were observed in the microcolumn, although many contrast lines related to interference appeared in the images; the contrasts were introduced as a result of the bending of the thin TEM sample. Not only bright-field STEM and TEM images but also dark-field TEM images were obtained, and no dislocations were observed in the microcolumn. However, in some other microcolumns, threading dislocations were observed. However, as the microcolumns possess the strong PL intensity comparable to that of HVPE-grown GaN, relatively few microcolumns can be considered to contain dislocations.
4. Conclusion 3.2. Emission properties of microcolumns Figs. 6(a) and (b) show an SEM top image and the corresponding CL image at the same area of the microcolumns (in sample C), respectively. The CL measurement was performed under electron-beam irradiation with an acceleration voltage of 5 kV. Three microcolumns of 2 mm diameter were observed in the SEM image; the CL from the corresponding three microcolumns was strong when compared with that from the area outside the microcolumns. The room-temperature PL property of the overgrowth sample with the microcolumns (sample C) was evaluated under He–Cd laser (325 nm) light irradiation. For comparison, HVPE-grown undoped GaN bulk crystal with a TDD of 6.0 106 cm 2 was also evaluated. Fig. 7 shows the PL spectra of a GaN microcolumn (from sample C) and the HVPE-grown GaN substrate. The peak wavelength of sample C was 364.7 nm, longer than that of the GaN substrate (363.8 nm), indicating the presence of residual tensile
GaN crystals were overgrown on Be-doped GaN nanocolumn platforms, which were prepared on an n-type (111) Si substrate. GaN overgrowth crystals grown with a low Ga-BEP of 1.0 10 4 Pa produce continuous films but with a high dislocation density and weak PL emission. Thus, the overgrowth of GaN on self-assembled nanocolumns is not a suitable method for the fabrication of GaN substrates with high crystal quality. In the overgrowth crystals grown under a higher Ga-BEP of 5.0 10 4 Pa, many micrometerscale hexagonal columnar crystals, named ‘microcolumns’, were observed. CL and PL measurements demonstrated the high optical properties of the microcolumns.
Acknowledgements This study is partly supported by a Grant-in-Aid for Scientific Research on Priority Areas #17656026 and (B) #18310079
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