Solution growth of GaN on sapphire substrate under nitrogen plasma

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 440–442

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Solution growth of GaN on sapphire substrate under nitrogen plasma Tetsuo Ozawa a,, Minoru Dohi a, Takeshi Matsuura a, Yasuhiro Hayakawa b a b

Department of Electrical, Electronic and Information Engineering, Shizuoka Institute of Science and Technology, Fukuroi 2200-2, Shizuoka 437-8555, Japan Research Institute of Electronics, Shizuoka University, Johoku 3-5-1, Hamamatsu 432-8011, Japan

a r t i c l e in f o

a b s t r a c t

Available online 24 September 2008

GaN layers were grown on (0 0 0 1) sapphire substrates by saturating Ga metal with atomic nitrogen and hydrogen mixture in a microwave plasma operating at 1000 Pa with the power of 380 W to avoid high equilibrium pressure and temperature environment. The semi-transparent GaN layer of about 3 mm was grown on the substrate at a growth rate of 0.6 mm/h under hydrogen flow ratio of 50%. The film had luminescence property without a deep level yellow emission. These results indicate that the direct conversion of Ga metal into GaN layer under nitrogen plasma is a useful method. & 2008 Published by Elsevier B.V.

PACS: 52.77. j 81.10. h Keywords: A2. Growth from solution A3. Liquid phase epitaxy B1. Nitride B2. Semiconducting gallium compounds

1. Introduction GaN-based materials are attractive for optoelectronic device applications such as light-emitting diodes (LEDs) and laser diodes (LDs) [1–3], as well as high-power high-frequency electronic devices including high electron mobility transistors (HEMTs) [4]. GaN substrates with low dislocation density is very important for improving device performance. It has been shown that substrates with dislocation density of approximately 102 cm 2 enabled fabrication of high-power LDs in which the total emitted power exceeds 3.9 W in 30 ns pulses [5]. Therefore, great effort has been taken to grow large GaN single crystals with low dislocation density, which could be used as a substrate. At present, solution growth methods like high-pressure solution growth [6], Na-based flux method [7–10] and lowpressure solution growth [11–15], are explored to grow GaN substrates. Our group has developed sub-atmospheric pressure solution growth in which gallium (Ga) metal reacts with atomic nitrogen in a microwave plasma to avoid the high equilibrium pressure and temperature environment [16]. Polycrystalline GaN can be synthesized at low equilibrium pressures of 200–400 Pa and low temperature of 610–700 1C. The yield of GaN reached 96% at 3 h. The growth of GaN layers on (0 0 0 1) sapphire substrate was carried out using N plasma. However, GaN hexagonal particles were formed with high nucleation density. The method to prevent formation of high nucleation density is required. The

paper demonstrates the influence of plasma mixture of nitrogen and hydrogen on the GaN crystal growth.

2. Experimental method GaN layer was grown on (0 0 0 1) sapphire substrate by saturating Ga metal with atomic nitrogen and hydrogen mixture in a microwave plasma (2.45 GHz) operating at 1000 Pa with the power of 380 W to avoid high equilibrium pressure and temperature environment. The 2 g Ga (6 N) in a pyrolytic boron nitride (PBN) crucible (10 mm diameter and 12 mm high) was initially evacuated to 1 10 4 Pa, and was heated at 610–700 1C in the growth chamber. Details of the procedure and setup are described in the previous paper [16]. The plasma mixture of nitrogen and hydrogen was exposed to Ga metal for 240–420 min. The flow rate of mixture gases was 100 cc/min and mixture ratios of hydrogen were 0–60%. The morphology and structure of the GaN layers were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The composition ratios of Ga and nitrogen in the layers were measured by the energy dispersive X-ray spectroscopy (EDS). The optical properties of the layers were characterized by photoluminescence (PL), where excitation source is 150 W xenon lamp and band pass filter is 10 nm.

3. Result and discussion Corresponding author. Tel.: +81 538 45 0151; fax: +81 538 45 0110.

E-mail address: [email protected] (T. Ozawa). 0022-0248/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2008.09.051

Fig. 1 shows photographs of the GaN layer grown on sapphire substrate by using plasma mixture of nitrogen and hydrogen,

ARTICLE IN PRESS T. Ozawa et al. / Journal of Crystal Growth 311 (2009) 440–442

3mm

441

3mm

3mm

Fig. 1. Photographs of the GaN layer grown on (0 0 0 1) sapphire substrate by using the plasma mixture of nitrogen and hydrogen, where hydrogen flow ratios of (a), (b) and (c) were 0%, 20% and 50%, respectively. In (c), about 60% of the substrate was covered by the thin layer.

5 μm

5 μm

20000 Al2O3 (0002)

Intensity (cps)

15000

10000 GaN (0002)

5000

0

10

20

30

40

50

60

70

80

2θ (°) Fig. 2. SEM images of GaN at (a) 20% and (b) 50% for hydrogen flow ratio. (c) XRD spectrum of the layer shown in (b).

where hydrogen flow ratios of (a), (b) and (c) were 0%, 20% and 50%, respectively. The plasma mixture was exposed to Ga metal at 1000 Pa with microwave power of 380 W for 300 min. In Fig. 1(a), the part of dark gray color on the sapphire substrate was a polycrystalline GaN. Fig. 1(b and c) shows that GaN layer of yellow color and semi-transparent layer were grown on the substrate, respectively. Fig. 2 show SEM images of GaN at (a) 20% and (b) 50% hydrogen flow ratio. In (a), the grown surface was rough. Composition of nitrogen and Ga measured by EDS were 34 and 66 at%, respectively. Many GaN crystals of cylindrical shape grew on the sapphire substrate. The size of particles ranged from 2 to 6 mm with a majority of the particles of about 4 mm. In (b), the grown surface was flat and about 60% of the substrate was covered by the thin layer. Nitrogen composition was close to stoichiometric value of GaN. The XRD spectrum of this layer is shown in Fig. 2(c). The orientation of GaN layer was (0 0 0 2),

which indicates that the single crystal was grown on the sapphire substrate. Fig. 3 show SEM images of the cleaved GaN layers grown on sapphire substrate, where (a) was grown with only nitrogen plasma for 300 min and (b) was the plasma mixture of hydrogen 50% and nitrogen 50% for 420 min. In (a), polycrystal with the thickness of about 4.3 mm was grown. The interface between GaN grown layer and substrate was rough and many voids were formed. The formation of the many voids is considered as follows: (1) nitrogen plasma and Ga metal reacts at the surface of Ga melt and Ga–N clusters are formed, (2) Ga–N clusters are transported to the bottom of the crucible by thermal convection flow in the Ga melt, and (3) GaN particles are deposited on the substrate. Since the nucleation density is too high, polycrystal with many voids is grown. In the case of (b), the interface shape and growth surface were flat and the average thickness of GaN layer was about 3 mm. The GaN grown layer has a strong (0 0 0 1) peak in the

ARTICLE IN PRESS T. Ozawa et al. / Journal of Crystal Growth 311 (2009) 440–442

GaN grown layer

2 μm

Substrate

Intensity (a.u.)

442

350

400

GaN grown layer

450 500 Wavelenghth (nm)

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Fig. 5. PL spectrum measured at room temperature, where hydrogen flow ratio is of 50% and nitridation time of 420 min.

2 μm

Fig. 3. SEM pictures of cleaved GaN layer grown on (0 0 0 1) sapphire substrate, where (a) only nitrogen plasma for 300 min and (b) plasma mixture of hydrogen and nitrogen for 420 min.

property suggests the growth of high-crystalline GaN. These results indicate that the direct conversion of Ga metal into GaN layer under nitrogen and hydrogen plasma mixture is an useful method.

Thickness of grown layer (μm)

2.0 4. Conclusion

1.5

We have carried out preliminary experiments to explore subatmospheric pressure solution growth to grow thick single GaN crystal films for the use as substrate. Semi-transparent GaN layer was grown on the (0 0 0 1) sapphire substrate by using plasma mixture of nitrogen and hydrogen. In comparison with only nitrogen plasma process, the growth morphology of the layer was improved. The luminescence property without a deep level yellow emission suggested the growth of high-crystalline GaN.

1.0

0.5

0.0 200

250

300 350 Nitridation time (min)

400

450

Fig. 4. Thickness of grown layer as a function of nitridation time, where hydrogen flow ratio is of 50%.

XRD in Fig. 2(c). It shows that the layer was nearly oriented to (0 0 0 1)GaNJ(0 0 0 1) sapphire substrate. The effect of plasma mixture is considered as follows. Hydrogen plasma removes Ga oxide from the surface of Ga melt, since nucleation density at the surface of Ga melt is reduced. The amount of dissolved atomic nitrogen is decreased compared with the case of 100% nitrogen plasma. As a result, the growth rate becomes low. The thickness of grown layer as a function of nitridation time is shown in Fig. 4, where hydrogen flow ratio is of 50%. The thickness of grown layer was increased with the increase of nitridation time from 240 min. The average growth rate was 0.6 mm/h. GaN layer was not grown on the substrate until 240 min. It suggested that it took time to transfer Ga–N cluster to the substrate. Fig. 5 shows PL spectrum measured at room temperature, where hydrogen flow ratio of 50% and nitridation time of 420 min. The spectrum showed a strong band edge emission at 3.46 eV without a deep level yellow emission. A full-width at halfmaximum was approximately 230 meV. This luminescence

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