7
34 Hetero-epitaxial Growth Mechanisms of AlN Single Crystals in Sublimation Growth Tomokazu Miura, 1 Tomohisa Kato, 1 Masashi Hatada, 2 Hideyuki Takashima, 2 Manabu Saito, 2 Kazuhiro Yamamoto, 3 Hiroyuki Kamata, 2 and Kunihiro Naoe 2 An aluminum nitride (AlN) single crystal is a promising material as a substrate for AlN- based semiconductors used in low-loss electronic devices and DUV emitting devices. We have developed a growth technology of AlN single crystals by a sublimation method which uses silicon carbide (SiC) as a seed. We observed the surface morphology focusing on the growth of the crystal at the initial stage. We have derived a novel model that explains the formation process of pyramids and the nucleation of AlN from the observation results. In addition, the apparent rotation of the facets of AlN islands with respect to the facets of pyramids is 30 degrees. However, it has been found that AlN and SiC had the same crystal orientation by crystallographic evaluations. 1. Introduction In recent years, as green innovation has been pro- moted in terms of energy savings and carbon dioxide reductions, a variety of technologies such as photovolta- ics, smart grids, and electric vehicles have emerged. Highly efficient technologies for converting and con- trolling electric power are indispensable to green inno- vation, for example low-loss and high-voltage power de- vices. Consequently, wide band gap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have attracted attention. These materials have a wider band gap than those of conventional semiconductors such as silicon (Si) and gallium arsenide (GaAs). Table 1 shows physical properties of important semiconduc- tors 1)-3) . AlN has the widest band gap among all semi- conductors, and the electrical breakdown strength of AlN is higher than that of SiC and GaN 4) . AlN is a promissing crystal to enable small, low-loss and high- voltage electronic devices and hetero-junction transis- tors using aluminium gallium nitride (AlGaN), which is a mixed crystal of AlN and GaN, have been already de- veloped 4) . Since the interband transition of nitride semicon- ductors such as AlN and AlGaN is a direct transition, the materials are suitable for luminescence devices in- cluding deep UV light emitting diodes (DUV-LEDs) and laser diodes (DUV-LDs). The DUV light 5) is ex- pected to be used for various applications such as de- composition of hazardous substance, sterilization, pu- rification, medical care and high-density optical recordings 6) . In general, sapphire is used as a substrate for AlN epilayer growth 7) . However, sapphire has problems such as high density defects due to a lattice mismatch between sapphire and AlN or AlGaN and warpage and crack due to large differences in their linear expansion coefficients. Therefore, AlN is expected to be one of the most appropriate substrate for the epilayer growth of AlN and AlGaN 8) because both a lattice mismatch between AlN and AlN or AlGaN, and the differences in their thermal expansion coefficients are relatively small. Today, high-quality and large-scale ingots such as Si and GaAs are produced by a liquid-phase method such as Czochralski (CZ) method. However, it is difficult for AlN to use a liquid phase method because the dis- sociation pressure of nitrogen is so high that vapor phase methods such as a sublimation method (Lely method) 9) or a hydride vapor phase epitaxy (HVPE) method 10)-12) are commonly used. The sublimation 1 : National Institute of Advanced Industrial Science and Technology, Advanced Power Electronics Research Center (ADPERC) 2 : New Technology Research Section, Environment and Energy Laboratory, Fujikura Ltd. 3 : Photovoltaics Research Section, Environment and Energy Laboratory, Fujikura Ltd. Table 1. Physical properties of important semiconductors. 1)-3) Material AlN Si GaAs 4H-SiC GaN Diamond Bandgap/eV 6.28 1.12 1.43 3.26 3.39 5.47 Electron mobility /cm 2 V -1 s -1 1,100 1,350 8,500 720 a /650 c 900 1,900 Electrical breakdown strength /10 6 V cm -1 11.7 0.3 0.4 2 3.3 5.6 Thermal conductivity /W cm -1 K -1 3.4 1.5 0.5 4.5 1.3 20 Band type D I D I D I a : a-axis direction, c : c-axis direction D : direct transition, I : indirect transition

Hetero-epitaxial Growth Mechanisms of AlN Single … · Hetero-epitaxial Growth Mechanisms of AlN Single Crystals in Sublimation Growth ... trolling electric power are indispensable

  • Upload
    vuongtu

  • View
    222

  • Download
    0

Embed Size (px)

Citation preview

34

Hetero-epitaxial Growth Mechanisms of AlN Single Crystals in Sublimation Growth

Tomokazu Miura,1 Tomohisa Kato,1 Masashi Hatada,2 Hideyuki Takashima,2 Manabu Saito,2 Kazuhiro Yamamoto,3 Hiroyuki Kamata,2 and Kunihiro Naoe2

An aluminum nitride (AlN) single crystal is a promising material as a substrate for AlN-based semiconductors used in low-loss electronic devices and DUV emitting devices. We have developed a growth technology of AlN single crystals by a sublimation method which uses silicon carbide (SiC) as a seed. We observed the surface morphology focusing on the growth of the crystal at the initial stage. We have derived a novel model that explains the formation process of pyramids and the nucleation of AlN from the observation results. In addition, the apparent rotation of the facets of AlN islands with respect to the facets of pyramids is 30 degrees. However, it has been found that AlN and SiC had the same crystal orientation by crystallographic evaluations.

1. IntroductionIn recent years, as green innovation has been pro-

moted in terms of energy savings and carbon dioxide reductions, a variety of technologies such as photovolta-ics, smart grids, and electric vehicles have emerged. Highly efficient technologies for converting and con-trolling electric power are indispensable to green inno-vation, for example low-loss and high-voltage power de-vices. Consequently, wide band gap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have attracted attention. These materials have a wider band gap than those of conventional semiconductors such as silicon (Si) and gallium arsenide (GaAs). Table 1 shows physical properties of important semiconduc-tors 1)-3). AlN has the widest band gap among all semi-conductors, and the electrical breakdown strength of AlN is higher than that of SiC and GaN 4). AlN is a promissing crystal to enable small, low-loss and high-voltage electronic devices and hetero-junction transis-tors using aluminium gallium nitride (AlGaN), which is a mixed crystal of AlN and GaN, have been already de-veloped 4).

Since the interband transition of nitride semicon-ductors such as AlN and AlGaN is a direct transition, the materials are suitable for luminescence devices in-cluding deep UV light emitting diodes (DUV-LEDs) and laser diodes (DUV-LDs). The DUV light 5) is ex-pected to be used for various applications such as de-composition of hazardous substance, sterilization, pu-rification, medical care and high-density optical

recordings 6).In general, sapphire is used as a substrate for AlN

epilayer growth 7). However, sapphire has problems such as high density defects due to a lattice mismatch between sapphire and AlN or AlGaN and warpage and crack due to large differences in their linear expansion coefficients. Therefore, AlN is expected to be one of the most appropriate substrate for the epilayer growth of AlN and AlGaN 8) because both a lattice mismatch between AlN and AlN or AlGaN, and the differences in their thermal expansion coefficients are relatively small.

Today, high-quality and large-scale ingots such as Si and GaAs are produced by a liquid-phase method such as Czochralski (CZ) method. However, it is difficult for AlN to use a liquid phase method because the dis-sociation pressure of nitrogen is so high that vapor phase methods such as a sublimation method (Lely method) 9) or a hydride vapor phase epitaxy (HVPE) method 10)-12) are commonly used. The sublimation

1 : National Institute of Advanced Industrial Science and Technology, Advanced

Power Electronics Research Center (ADPERC)2 : New Technology Research Section, Environment and Energy Laboratory,

Fujikura Ltd.3 : Photovoltaics Research Section, Environment and Energy Laboratory,

Fujikura Ltd.

Table 1. Physical properties of important semiconductors. 1)-3)

Material AlN Si GaAs 4H-SiC GaN Diamond

Bandgap/eV 6.28 1.12 1.43 3.26 3.39 5.47

Electron mobility

/cm2 V-1 s-1

1,100 1,350 8,500 720a/650c 900 1,900

Electrical breakdown

strength/106 V cm-1

11.7 0.3 0.4 2 3.3 5.6

Thermal conductivity/W cm-1 K-1

3.4 1.5 0.5 4.5 1.3 20

Band type D I D I D I

a : a-axis direction, c : c-axis directionD : direct transition, I : indirect transition

Fujikura Technical Review, 2014 35

method is a growth method in which a source material is sublimated at high temperatures, and the vapor be-comes supersaturated and is recondensed as a crystal at lower temperatures. A seeded sublimation method is called as the modified Lely method. The modified Lely method 13)-18) can control nucleation, so there is a merit to grow large-sized crystals with relative ease.

We have adopted the modified Lely method for AlN single crystal growth. SiC is chiefly used as a seed for AlN sublimation growth because a lattice mismatch between SiC and AlN is small and the vapor pressure of SiC is smaller than that of AlN at growth tempera-tures of AlN. Controlling the initial growth is very im-portant to decrease defects in the hetero epitaxial growth of the crystal, and various researches on this matter have been reported 14)-18).

Y. Shi et al. stated that hexagonal pyramids were formed on an SiC seed with AlN rotated by 30 degrees and grown on the apexes of the pyramids. They also stated the analysis results of the Kikuchi patterns ob-served in EBSD. According to the results, [0 0 0 2]AlN was parallel to [0 0 0 6]sic while [1 1 -2 0] was rotated by 30 degrees with respect to [1 1 -2 0]sic 14). G.R. Yazdi et al. investigated the formation of the pyramids and the growth mechanism of the crystal grown on the apexes of them. They stated that pyramids were grown out of hexagonal pits formed by thermal etching of the seed during temperature rise. The polytype of pyramids on 4H-SiC was 2H 15)16). R. R. Sumathi et al. discussed the crystalline of AlN islands grown on pyramids 17). C. Hartmann et al. observed that SiC pyramids were formed only on the (0 0 0 1) Si plane of SiC, and they stated that the main cause was anisotropic oxidation of SiC by Al2O(g) which was formed during heating up in the AlN source 18).

As mentioned above, there are various explanations for the AlN growth. However, the orientation between AlN and SiC, and the formation process of SiC pyra-mids are still unclear. The purpose of this paper is to solve these two questions. We have verified the forma-tion process of pyramids by the surface analysis and

the section observations at the initial growth stage. In addition, we measured the crystallographic orienta-tion of SiC and AlN and observed the surface structure of SiC using a field emission scanning electron micro-scope (FE-SEM). The results are as follows.

2. ExperimentsAlN single crystals were grown on the (0 0 0 1) Si

plane of 4H- and 6H-SiC seeds by a sublimation meth-od using a furace heated by a high-frequency induc-tion coil wound around the crucible. The crystal growth system is shown in Fig. 1. The crucible was made of graphite or tungsten. The temperatures were monitored at the top and the bottom of the crucible by a radiation thermometer through a small hole in the thermal insulator covering the crucible. The coil was positioned to adjust a temperature gradient between the source material and the seed. Crystal growth was performed at temperatures between 1,700 and 2,025 ∞C in a pure nitrogen atmosphere at pressures be-tween 13 and 101 kPa for 0 to 6 hours. When a graphite crucible was used, a tantalum carbide crucible was used as an inner crucible. This double crucible pre-vented AlN crystals from containing impurities.

Surfaces and cross sections of the samples were ob-served by a scanning electron microscope (SEM), op-tical microscope and FE-SEM. The elements of each crystal were analyzed by micro Raman scattering mea-surements and Energy Dispersive x-ray Spectroscopy (EDS). Micro Raman scattering measurements were performed using laser light of a wavelength of 532nm, and the irradiation direction was parallel to the growth direction. In order to investigate the epitaxial relation-ship between SiC and AlN, electron backscatter dif-fraction (EBSD) measurement was performed at a cross section. Phi scans and pole figure measurements were performed using X ray diffraction equipment for

Crucible

Insulator

Coil

SiCAlN

AlN Souce

Fig. 1. Schematic illustration of the crystal growth system.

(a) (a)

(c) (d)

Fig. 2. SEM images of pyramids formed on SiC (0 0 0 1) substrates at initial growth.

36

the confirmation of crystal orientations. Gold was evaporatively deposited on the samples for observing with a SEM, FE-SEM, EDS and EBSD for conduction of electricity. The CuKa rays were used as a radiation source.

3. Results and discussion3.1 Formation of pyramids and nucleation of AlN3.1.1 Surface observations after 6 hr growth

The nucleation of AlN crystals was observed after 6 hours of crystal growth on an SiC substrate. Figure 2 shows SEM images of SiC at the initial growth stage. As shown in Fig. 2, pyramids of 15 to 200 μm in diago-nal, 10 to 150 μm in length were formed on the SiC seed after 6 hr growth. A minute hexagonal island or a hexagonal pyramid also grows on the vertex of a pyra-mid. For identification of these phases and structures, EDS measurements and micro Raman scattering mea-

surements were performed. Figure 3 and 4 show the results of EDS and Raman scattering spectrum mea-surements. As shown in Fig. 3, it is confirmed that these pyramids consisted of SiC and the islands on the vertex of pyramids consisted of AlN. AlN grown in the initial stage included a large amount of Si generated by the decomposition of SiC. As shown in Fig. 4, the poly-type of pyramids is in agreement with the polytype of the seeds. The result of Fig. 4 differs from that report-ed by G.R. Yazdi et.al. 14)15). Moreover, the peak which should not appear in the Raman selection rule ap-peared. This may indicate the crystal plane of the pyra-mids tilted. Both SiC and AlN have hexagonal crystal structure, so each hexagonal facet observed in the pyramids and the islands are typical of the substances. AlN islands have facets parallel to the surface of the SiC seed, so it seems that lateral growth is preferential at the initial stage of growth.

1000900800700600500400300200100

0

Counts

10.00

Al

AuAu Au Au

Si

Au 0.1 mm

(b)

0.001.00

2.003.00

4.005.00

keV

6.007.00

8.009.00

10.00

(a)keV

1000

Cou

nts

900

12001100

800700600500400300200100

0

1.002.000.00

3.004.00

5.006.00

7.008.00

9.00

Si

AuC

AuAu AuAu20 µm

Fig. 3. EDS of (a): a pyramidal hillock and (b): a island grown on the pyramidal hillock.

E2(PO)

E1(TO)

LOPC

E2(PA)

E1(PA)

A1(AA)

pyramids on a 4H-SiC substrate

Raman shift/cm-1Raman shift/cm-1

E2(PO)

A1(LO)

E2(PA)

E1(PA)A1(AA)

pyramids on a 6H-SiC substrate

E2(PA)

(a) (b)

500 1000

Inte

nsity

/a.u

.

500 1000

Inte

nsity

/a.u

.

Fig. 4. Raman scattering measurements of the pyramids grown on (a): 4H-SiC substrate and (b): 6H-SiC substrate.PA: Planar acoustic mode, AA: Axial acoustic mode, PO: Planar optic mode, TO: Transvers optical mode,

LOPC: Longitudinal optical phonon plasmon coupring mode

Fujikura Technical Review, 2014 37

3.1.2 Surface observations after 0 hr growth

A short-time (almost 0 hr) growth experiment was conducted to investigate the formation process of SiC pyramids. Figure 5 shows FE-SEM images of SiC be-fore the formation of SiC pyramids is completed. Al-though AlN islands were observed in the 0 hr growth stage, the formation of pyramids was not completed, which allowed observation of the formation process. Fig. 5 (a) and (b) show linear grooves which were probably etched with sublimated AlN and were gener-ated on the surface of the SiC seed. Fig. 5(a) and (b) also show the side walls of the grooves which were the walls of the pyramids. The sidewalls of the pyramids lined up in the grooves. This means that the pyramids consist of SiC which have remained without being etched. As shown in Fig. 5(c), (d), it was also observed that AlN grew on an SiC seed surface which had not been etched yet. The nucleation site of AlN corre-sponded to the formation site of SiC pyramids. AlN is-lands were also observed on the apexes of SiC pyra-mids. Nucleation was not observed at the flat area made by etching.

Figure 6 shows the transmission optical microscope images of the SiC (0 0 0 -1) C plane which was the op-posite side of the plane where pyramids were formed. It was confirmed that the pits were made in the SiC (0 0 0 -1) C plane, and the pyramids on the SiC (0 0 0 1) Si plane were connected to the pits by line defects.

Generally, the nucleation occurs at preferential sites such as crystal grain boundaries and dislocations. As-suming that threading line defects exist in the SiC pyramids, it is no wonder that the apexes of SiC pyra-mids work as nucleation sites.

Therefore, it has turned out that pyramids are formed by the etching of SiC seeds by AlN sublimates leaving {1 1 -2 m} planes. Moreover, since a threading line defect exists at the pyramid, it seems that the ver-tex of a pyramid is easy to work as a preferential nucle-ation site.

3.1.3 Lateral growth of AlN islands

Figure 7 shows SEM images in the second stage of growth. In additional lateral growth, AlN islands co-alesce to form a layer. Each hexagonal AlN island grown on SiC has the same orientation, so it is consid-ered that AlN epitaxial growth occurs on the SiC. Al-though the growth occurs by multiple nucleation, the islands coalesce and grow as a single crystal, because the islands have the same orientation by epitaxial growth.

3.2 Crystallogic Orientation Relationship between SiC and AlN

Figure 8 shows SEM images of the positional rela-tionship between the hexagonal AlN islands and hex-agonal SiC pyramids. AlN islands were sixfold sym-

SiC

AlN

AlN

SiC

AlN

Fig. 7. Lateral growth process of AlN.

0.200 mm 0.200 mm

Fig. 6. Line defects observed at pyramids.

30°

Fig. 8. SEM images of AlN crystals grown on SiC pyramids. In spite of the smaller lattice mismatch between AlN and SiC, the AlN islands are rotated by 30 degrees with respect to the SiC

pyramids.

AlN

AlN

AlN

SiCSiC

SiC pyramids

SiC

(a) (b)

(c) (d)

Fig. 5. Formation process of SiC pyramids and nucleation of AlN.

38

metric around c-axis, however, the hexagonal AlN islands rotated by 30 degrees with respect to the hex-agonal SiC pyramids. As shown in Fig. 9, the steps of SiC also appeared rotated by 30 degrees with respect to the hexagonal SiC pyramids, so the steps of SiC were parallel to the hexagonal AlN islands. It was un-derstood that the steps were parallel to {1 0 -1 0}, that SiC pyramids had {1 1 -2 m}facets, and AlN islands had {1 0 -1 n} facets, through checking the m-plane orienta-tion flat of the substrate. Therefore, in terms of mor-phology, SiC and AlN single crystal had the same ori-entation.

Phi scans and pole figure measurements were per-formed for the confirmation of the crystal orientation. The orientations of AlN and SiC seeds were deter-mined by Phi scans of (1 0 -1 2) and (1 1 -2 2) reflec-tions of AlN and (1 0 -1 2), (1 1 -2 4) and (1 0 -1 4) re-flections of SiC. The results are shown in Fig. 10 and Fig. 11. 4H-SiC (1 1 -2 4) and AlN (1 1 -2 2) had the ac-cumulation of poles in the same phi position, and 4H-SiC (1 0 -1 4), 4H-SiC (1 0 -1 2), and AlN (1 0 -1 2) had the accumulation of poles in the same phi position. Therefore, (1 0 -1 0) AlN was parallel to (1 0 -1 0) SiC and (1 1 -2 0) AlN was parallel to (1 1 -2 0) SiC. Just as

we estimated, the crystals had the same orientation. It is confirmed that the phi position with the accumula-tion of poles was the same also in pole figure measure-ments. So the orientations of SiC and AlN were the same. The orientations of SiC and AlN were measured by EBSD at the cross section. The result is shown in Fig. 12. Just as we thought, the orientation was the same in SiC and AlN.

As mentioned above, the epitaxial growth of AlN on SiC was observed. The apparent facet of AlN was {1 0 -1 n} and the apparent facet was {1 1 -2 m}, so the hexagonal islands of AlN and SiC pyramids apparently rotated by 30 degrees with respect to each other.

3. 3 Observations of the surface structure of pyra-mids

Figure 13 shows FE-SEM images of SiC pyramids. Minute protrusions with an angle of 120 degrees were observed on the surface of SiC pyramids, and it is un-derstood that the {1 1 -2 m} facet which forms the SiC pyramids consists of the steps parallel to {1 0 -1 0} planes. Therefore, the {1 1 -2 m} facet formed pyra-mids rotated by 30 degrees with respect to the {1 0 -1 n} facets of AlN islands, but the minute facets of

4H-SiC(10-14)4H-SiC(11-24)4H-SiC(10-12)AIN(11-22)AIN(10-12)

Phi/deg.90 1800 270 360

Inte

nsity

/a.u

.

Fig. 10. Phi scan measurements using (1 0 -1 2) and (1 1 -2 2) reflections of AlN and (1 0 -1 2), (1 1 -2 4) and (1 0 -1 4)

reflections of SiC.

(a)

(b)

Pole figure: 102 Raw2Theta: 38.2355Intensities:

Intensity0.000

487.000Dimension: 2DProjection: SchmidtScale: LinearColor map: X’PertContours: 10

Intensity44.273

132.818221.364309.909442.727

Grid settings:

FirstLastStep

Min 0.0 12.5Max 60.0 7.5

Psi Phi

Psi Phi

Color1357

10

900 0

36030 90

Pole figure: 102 Raw2Theta: 49.8013Intensities:

Intensity0.000

2717.000Dimension: 2DProjection: SchmidtScale: LinearColor map: X’PertContours: 10

Intensity

Grid settings:

FirstLastStep

Min 0.0 7.5Max 45.0 127.5

Psi Phi

Psi Phi

Color1357

10

900 0

36030 90

247.000741.000

1235.0001729.0002470.000

Fig. 11. {1 0 -1 2} Pole figures of (a) SiC and (b) AlN were drawn by using each (1 0 -1 2) diffraction. A project plane is a

plane vertically growth direction.

<11-20>

<10-10>

<11-20>

Fig. 9. SEM images of the steps on a SiC substrate and SiC pyramids. The Steps appeared rotated by 30 degrees with

respect to SiC pyramids.

Fujikura Technical Review, 2014 39

SiC pyramids aligned parallel to the {1 0 -1 n} facets of AlN islands. Moreover, the center of the facet which forms the pyramid ({1 1 -2 m} <1 1 -2 n>) had larger terraces but less in number compared to those in the sides. It is unknown why AlN rotates by 30 degrees from the hexagonal SiC pyramids. However, the cause may be attributed to the difference of surface energy anisotropy between SiC and AlN. It has been reported that the surface energy of m-plane {1 0 -1 0} of SiC is higher than that of a-plane {1 1 -2 0}, so m-plane {1 0 -1 0} is less stable than a-plane {1 1 -2 0} 19). It corresponds to a fact that the SiC pyramids consist of pyramidal plane of {1 1 -2 m} which is inclined a-plane.

4. ConclusionWe grew AlN single crystals on SiC seeds by subli-

mation method for a very short time, observed them, and measured the crystallogic orientation to investi-gate the pyramid formation process and the orienta-tion of SiC and AlN. The results are summarized in the paragraphs below.1 SiC pyramids are formed by etching of an SiC seed

leaving {1 1 -2 m} facets with AlN sublimates. Since the threading line defect exists in the SiC pyramids, the vertex of a pyramid probably tends to work as a preferential nucleation site.

2 The hexagonal AlN islands are rotated by 30 degrees with respect to the hexagonal SiC pyramids. How-ever, it has been found that the facets are {1 0 -1 n}

for AlN and {1 1 -2 m} for SiC, respectively. There-fore, they have an identical orientation.

3 {1 1 -2 m} of SiC forms pyramids shifted by 30 de-grees from the {1 0 -1 n} of AlN islands, but the min-ute facets of SiC pyramids aligne parallel to the {1 0 -1 n} facets of AlN islands. This paper has proposed a novel model of the forma-

tion process of pyramids in AlN crystal growth. An epi-taxial lateral overgrowth (ELO) method is often used for defective control 20). If the size, the density and the distribution of SiC pyramids can be controlled, we will produce high quality crystals that are comparable to those produced by the ELO method.

References

1) T. P. Cho, Mater. Sci. Forum, vol.338/342, p.1155, 20002) J. C. Rojo, L. J. Schowalter, K. Morgan, D. I. Florescu, F. H.

Pollak, B. Raghothamachar, M. Dudley, Mater. Sci. Res. Soc. Symp. Proc., vol.680E, E2.1.1, 2001

3) P. B. Perry, RF. Ruz, Appl. Phys. Lett., vol.33, p.319 19784) T. Nanjo, M. Takeuchi, M. Suita, T. Oishi, Y. Abe, Y. Tokuda,

and Y. Aoyagi: “ Remarkable breakdown voltage enhance-ment in AlGaN channel high electron mobility transistors”, Appl. Phys. Lett. 92, 263502, 2008

5) Y. Taniyasu, M. Kasu and T. Makimoto: “An aluminium ni-tride light-emitting diode with a wavelength of 210 nanome-tres”, Nature, vol.441 pp.325-328, 2006

6) Hideki. Hirayama, Sachie Fujikawa, Yusuke Tsukada, Nori-hiko Kamata, J. Appl. Phys. 80, No.6, pp.319-324, 2011

7) Hideki Hirayama, Sachie Fujikawa, Norimichi Noguchi, Jun Norimatsu, Takayoshi Takano, Kenji Tsubaki, and Norihiko Kamata: “222-282 nm AlGaN and InAlGaN-based deep-UV LEDs fabricated on high-quality AlN on sapphire”, Phys. Sta-tus Solidi A 206, No.6, p.p.1176-1182, 2009

8) R. Dalmau, S. Craft, B. Moody, R. Schlesser, S. Mita, J. Xie, R. Collazo, A. Rice, J. Tweedie, and Z. Sitar: “Challenges in AlN Crystal Growth and Prospects of the AlN-based Technol-ogy”, CS MANTECH Conference, May 16th-19th, 2001, Palm Springs, California, USA

9) G. A. Slsck and T.F. McNelly: “AlN single crystals”, J. Crysta. Growth, 34, 263, 1976

Aluminum Nitride(H)

SiC

AlN

AlN

SiC0001 2110

1010

Silicon Carbide 6H

0001 2110

1010

70 µm

Fig. 12. The EBSD image of a cross section of SiC and AlN.

{11-2m}

<10-10>

<11-20>

Fig. 13. FE-SEM images of a SiC pyramid. The minute protrusions with the angle of 120 degrees were observed on the

surface of a SiC pyramid.

40

10) Yusuke Katagiri, Shinya Kishino, Kazuki Okuura, Hideto Miyake, Kazumasa Hiramatu: “Low-pressure HVPE growth of crack-free thick AlN on a trench-patterned AlN template,” Journal of Crystal Growth, 311, p.p.2831-2833, 2009

11) Jie-Jun Wu, Yusuke Katagiri, Kazuki Okuura, Da-Bing Li, Hideto Miyake, Kazumasa Hiramatsu: “Effects of initial stages on the crystal quality of nonpolar a-plane AlN on r-plane sapphire by low-pressure HVPE,” Journal of Crystal Growth, 311, p.p.3801-3805, 2009

12) Takuya Nomura, Kenta Okumura, Hideto Miyake, Kazu-masa Hiramatsu, Osamu Eryu, Yoichi Yamada: “AlN ho-moepitaxial growth on sublimation-AlN substrate by low-pressure HVPE,” Journal of Crystal Growth, 350, p.p.69-71, 2012

13) Hiroyuki Kamata, Kunihiro Naoe, Kazuo Sanada and No-boru Ichinose: “Single-crystal growth of aluminum nitride on 6H-SiC seed crystals by an open-system sublimation method”, Journal of Crystal Growth Vol. 311(5), pp.1291-1295.

14) Y. Shi, B. Liu, J. H. Edgar, H. M. Meyer III, E. A. Payzant, L. R. Walker, N. D. Evans, J. G. Swadener, J. Chaudhuri and Joy Chaudhuri: “Initial Nucleation Study and New Tech-nique for Sublimation Growth of AlN on SiC Seed crystal,” Phys. Stat. Sol., (a) 188, No.2, pp757 -762, 2001

15) G.. R. Yazdi, M. Beckers, F. Giuliani, M. Syvajarvi, L. Hult-man, and R. Yakimova: “Freestanding AlN single crystals enabled by self-organization of 2H-SiC pyramids on 4H-SiC substrates,”Appl. Phys. Lett. 94, 082109, 2009

16) G.R. Yazdi, M. Syvajarvi, R. Yakimova: “Formation of nee-dle-like and columnar structures of AlN,” Jornal of Crystal Growth, 300, pp130-135, 2007

17) R. R. Sumathi, R. U. Barz, P. Gille, and T. Straubinger: “In-fluence of interface formation on the structural quality of AlN single crystals grown by sublimation method,” phys. status solidi C, 1-3, 2011

18) C. Hartmann, M. Albrecht, J. Wollweber, J. Schuppang, U. Juda, C. Guguschev: “SiC seed polarity-dependent bulk AlN growth under the influence of residual oxgen,” Journal of Crystal Growth, 2012

19) K. Togase, K. Terao, S. R. Nishitani, T. Kaneko and N.Ohtani: “First-principle calculation on Surface Energy of SiC”, Kwansei Gakuin University, CALPHAD XXXVIII (Prague, Czech Republic), 09/May/18

20) Hideki Hirayama, Sachie Fujikawa, Jun Norimatsu, Ta-kayoshi Takano, Kenji Tsubaki, and Norihiko Kamata: “Fabrication of a low threading dislocation density ELO-AlN template for applidation to deep-UV LEDs”, Phys. Status So-lidi C6, No.52, S359-S359, 2009