Homoepitaxial HVPE growth on GaN wafers manufactured by the Na

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Homoepitaxial HVPE growth on GaN wafers manufactured by the Na-flux method Masayuki Imanishi, Takehiro Yoshida, Toshio Kitamura, Kosuke Murakami, Mamoru Imade, Masashi Yoshimura, Masatomo Shibata, Yoshiyuki Tsusaka, Junji Matsui, and Yusuke Mori Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Crystal Growth & Design

1

Homoepitaxial HVPE growth on GaN wafers

2

manufactured by the Na-flux method

3

Masayuki Imanishi1*, Takehiro Yoshida2, Toshio Kitamura2, Kosuke Murakami1, Mamoru Imade1,

4

Masashi Yoshimura1, Masatomo Shibata2, Yoshiyuki Tsusaka3, Junji Matsui4, Yusuke Mori1

5 6

1

Division of Electric, Electronic and Information Engineering, Osaka University, 2-1 Yamada-oka, Suita-shi, Osaka 565-0871, Japan 2

7 8 9

3

10 11

4

SCIOCS, 880 Isagozawa-cho, Hitachi-shi, Ibaraki 319-1418, Japan

Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan

Synchrotron Radiation Nanotechnology Center, University of Hyogo, 1-490-2, Kouto, Shingu-cho, Tatsuno-shi, Hyogo 679-5165, Japan

12

ABSTRACT. Homoepitaxial HVPE growth on GaN substrates grown with a Na-flux method, which

13

is the most promising approach for fabrication of large-diameter, low-dislocation-density,

14

fast-growing GaN wafers, was attempted for the first time. We found that, when different growth

15

methods are combined, the differences in oxygen concentrations between a seed and grown crystal

16

must be eliminated to maintain the crystallographic quality of the seed. Two kinds of Na-flux-grown

17

seed crystals were prepared; one had a surface composed of c, {101ത2}, and {101ത1} planes, the other

18

a surface composed entirely of c-planes. Both crystals were sliced, ground, mirror-polished and

19

applied for 500-um-thick HVPE growth. In the former sample, the seed crystal generated fine cracks,

20

and the epitaxially grown layer had a rough surface and included many dislocations; the latter

21

sample showed no fault. For clarifying the mechanism of crystal degradation, we investigated the

22

lattice constants of each growth sector using an X-ray microbeam, and found that lattice constants in

23

the {101ത1}-growth sector were expanded compared to those in other growth sectors due to oxygen

24

impurities. These values were estimated to be much larger than those of HVPE crystals, resulting in

25

the crystal degradation after the HVPE growth by a lattice mismatch. 1

ACS Paragon Plus Environment


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*Corresponding author: Masayuki Imanishi

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Phone/Fax: +81-6-6879-7706 / +81-6-6879-7708

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E-mail: [email protected]

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2


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1

Homoepitaxial HVPE growth on GaN wafers

2

manufactured by the Na-flux method

3

Masayuki Imanishi1*, Takehiro Yoshida2, Toshio Kitamura2, Kosuke Murakami1, Mamoru Imade1,

4

Masashi Yoshimura1, Masatomo Shibata2, Yoshiyuki Tsusaka3, Junji Matsui4, Yusuke Mori1

5

1

6

Suita-shi, Osaka 565-0821, Japan

7

2

SCIOCS, 880 Isagozawa-cho, Hitachi-shi, Ibaraki 319-1418, Japan

8

3

Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun,

9

Hyogo 678-1297, Japan

Division of Electric, Electronic and Information Engineering, Osaka University, 2-1 Yamada-oka,

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4

11

Tatsuno-shi, Hyogo 679-5165, Japan

12

Phone: +81-6-6879-7706

13

FAX: +81-6-6879-7708

14

E-mail: [email protected]

Synchrotron Radiation Nanotechnology Center, University of Hyogo, 1-490-2, Kouto, Shingu-cho,

15

16

ABSTRACT. Homoepitaxial HVPE growth on GaN substrates grown with a Na-flux method, which

17

is the most promising approach for fabrication of large-diameter, low-dislocation-density,

18

fast-growing GaN wafers, was attempted for the first time. We found that, when different growth

19

methods are combined, the differences in oxygen concentrations between a seed and grown crystal

20

must be eliminated to maintain the crystallographic quality of the seed. Two kinds of Na-flux-grown 3



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1

seed crystals were prepared; one had a surface composed of c, {101ത2}, and {101ത1} planes, the other

2

a surface composed entirely of c-planes. Both crystals were sliced, ground, mirror-polished and

3

applied for 500-um-thick HVPE growth. In the former sample, the seed crystal generated fine cracks,

4

and the epitaxially grown layer had a rough surface and included many dislocations; the latter

5

sample showed no fault. For clarifying the mechanism of crystal degradation, we investigated the

6

lattice constants of each growth sector using an X-ray microbeam, and found that lattice constants in

7

the {101ത1}-growth sector were expanded compared to those in other growth sectors due to oxygen

8

impurities. These values were estimated to be much larger than those of HVPE crystals, resulting in

9

the crystal degradation after the HVPE growth by a lattice mismatch.

10 11

Introduction

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GaN substrates are generally regarded as ideal substrate for high-performance optoelectronic1,2

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and electronic power devices.3,4 For the widespread use of the GaN devices to occur, a reduction in

14

the cost of GaN substrates is strongly needed, particularly considering future reductions in the cost

15

of high-brightness LEDs and power devices. Toward that goal, enlargement of the wafer diameter

16

and rapid growth of thick GaN followed by wafer slicing are expected to be useful way for mass

17

production and for the reduction of manufacturing costs. Fabrication of bulk GaN has been

18

attempted using several growth techniques: the hydride vapor phase epitaxy (HVPE) method,5,6 the

19

ammonothermal method,7,8 the high nitrogen pressure solution method,9 and the Na-flux method.10,11

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However, there is as yet no way to fabricate bulk GaN that achieves low dislocation density ( 4 inches), low curvature (radius of curvature >20 m), and a high growth rate

22

(> 1 mm/h). Sochacki et al. demonstrated HVPE growth on ammonothermal GaN seed crystal.12

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With this method, GaN with a 1-inch-diameter and a low curvature was successfully grown. Tsukada

24

et al. succeeded in fabricating a 2-inch-diameter m-plane GaN wafer using HVPE growth using an

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acidic ammonothermal method called SuperCritical Acidic Ammonia Technology (SCAATTM ).13

26

However, enlargement of the diameter of the c plane, by more than 2 inches, is desired. 4


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We have attempted to enlarge the diameter of a GaN wafer using the multi-point seed (MPS)

2

technique, in which a sapphire substrate naturally separates in the cooling process after Na-flux

3

growth.10 With this technique, we successfully obtained a large-diameter (more than 4 inches)

4

freestanding GaN wafer.11 The threading dislocation density (TDD) of the wafer was on the order of

5

102 cm-2 to 105 cm-2, and the radius of lattice curvature is more than 100 m. However, the growth

6

rate along the c axis was 50 µm/h, which is still low for the mass production, and further

7

improvement of the growth rate is necessary. On the other hand, ultra-high-speed growth along the c

8

axis, more than 1 mm /h, was accomplished with HVPE growth by Yoshida et al.14 Thus, in the

9

present study, we propose HVPE growth on the Na-flux-grown wafer, that is, a hybrid growth

10

method. This is the most promising approach to fabricating a large-diameter, low-TDD, and

11

fast-growing GaN wafer. In this paper, we report on the first trial of HVPE growth on a

12

Na-flux-grown GaN wafer to verify that the seed’s high crystalline quality can be reproduced after

13

HVPE growth without generating defects at the interface.

14 15

Experimental Section

16

Two kinds of freestanding GaN wafer grown by the Na-flux method adopting multi-point seeds

17

technique, 10 as shown in Figs. 1a and 1b, were prepared as seeds for HVPE growth. One seed crystal

18

(Fig. 1a) consists of c, {101ത2}, and {101ത1} planes (seed A) as shown in the bird’s-eye SEM image of

19

the surface in Fig. 1c, and the other (Fig. 1b) consists of only a c plane (seed B) as shown in the

20

SEM image in Fig. 1d. Orientations of those planes were determined by measuring the inclination

21

angles to c planes, using cross sectional SEM images in the inset of Fig 1c and 1d. The average

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thickness was 1.4 mm (from 1.0 mm at the valley to 1.7 mm at the top) in seed A, and 1.2 mm

23

without variation in seed B. Seed A was grown under a conventional-supersaturation condition,

24

whereas seed B was grown under the higher-supersaturation condition which promotes the lateral

25

growth rate perpendicular to the c axis, resulting in the expansion of c-plane area. Seed A was

26

ground to remove the as-grown surface roughness and make it flat. Chemical mechanical polishing 5



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(CMP) was then conducted to eliminate the induced surface damage layer. The surface of seed B

2

was also treated with CMP to make the surface conditions uniform for both seeds. The size and

3

shape of the seed A was a 2-inch-diameter circle (Fig. 1e), while seed B was a 20 mm × 20 mm

4

square (Fig. 1f). The thicknesses of seed A and B were 700 µm and 400 µm, respectively.

5

GaN crystals around 500-µm thick were grown homoepitaxially on freestanding GaN substrates

6

grown by the Na-flux method, using a conventional atmospheric HVPE reactor.15 The HVPE growth

7

rate was around 200 µm/h. The structural quality of the crystals after HVPE growth was assessed by

8

evaluating of the surface morphology, the TDD, and the radius of lattice curvature of the c-plane

9

(Rc). The surface and back side morphology was observed by differential interference contrast (DIC)

10

microscopy and secondary electron microscopy (SEM), respectively. TDD was calculated from the

11

etch-pit density observed using SEM after the KOH-NaOH melt etching at 450°C for 30 min. Rc

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was measured via X-ray diffraction (XRD) on five regions with 4 mm spacing using a Rigaku

13

Smartlab x-ray diffractometer. Rc was then determined by performing on-axis (0002) omega scans,

14

then translating the sample and measuring the displacement of the omega peak positions. The details

15

were described in a previous paper.10

16

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Figure 1. Photo images of as-grown GaN crystals grown with the Na-flux method, which consist of

3

(a) {101ത1}, {101ത2}, and c planes (named seed A) and (b) only c planes (named seed B), and

4

bird’s-eye SEM images of the surface of (c) seed A with the scheme describing the crystal-plane

5

orientations at the upper right and the surface of (d) seed B showing an entirely flat c plane. Cross

6

sectional SEM images for determining orientation of planes were in set of Fig 1c and 1d. Photo

7

images after dicing and grinding of the surface of as-grown (e) seed A and (f) seed B.

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Crystal Growth & Design 1

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Results and discussion

2

Optical photo images of the as-grown GaN crystals after the HVPE growth on seeds A and B are

3

depicted in Figs. 2a and 2b, respectively. We named the HVPE crystal on seeds A and B as samples

4

A and B, respectively. GaN crystals were successfully grown on all of the area with specular surfaces

5

in each samples. Many grown-in-pits (more than 100) appeared on the surface of sample A, around

6

the area indicated with white arrows in Fig. 2a, while only 7 pits were observed on the surface of

7

sample B, as individually pointed with white arrows in Fig. 2b. Some areas in sample A showed

8

metallic luster of gallium due to the decomposition of GaN crystals. These were attributed to the

9

generation of cracks parallel to the c plane, which caused a lack of nitrogen sources on crystal

10

surfaces under a high temperature-condition, and induced the decomposition of GaN crystals.

11

Figures 2c and 2d also show surface images of samples A and B taken with DIC, respectively.

12

Sample A’s surface showed a high density of hillocks, while sample B showed no hillocks and an

13

entirely smooth surface. Many cracks were also found in the Na-flux-seed crystal of sample A in a

14

backside SEM image depicted in Fig. 3a, whereas no cracks were observed in sample B as depicted

15

in Fig 3b.

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Figure 2. Photo images of as-grown GaN crystals after HVPE growth on (a) seed A (named sample

3

A) and (b) seed B (named sample B). White arrows indicate grown-in pits observed on the surfaces

4

of both crystals. The surface morphologies of (c) sample A and (d) sample B, observed by

5

differential interference contrast (DIC) microscopy.

6

7 8

Figure 3. Backside SEM images of (a) sample A, showing some cracks in the seed A, and (b)

9

sample B, showing no cracks in the seed B. 9



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1

Secondly, the TDD was investigated using KOH-NaOH etching. Figure 4a and 4b show surface

2

SEM images after the etching on the HVPE layers, with several etch pits arising from threading

3

dislocations (TDs). The TDD of sample A was more than 1×107 cm-2, which is much higher than that

4

of seed A. In contrast, a few TDs were observed on the surface of sample B; the TDD value was on

5

the order of 104 to 105 cm-2, which is as much as that of seed B. These results indicate that TDs were

6

generated at the epi−sub interface during HVPE growth on seed A, whereas TDs in seed B

7

propagated without increasing during HVPE growth. It is speculated that TDs in sample A were

8

involved in the development of the hillocks on the surface. Rc values of both the grown crystals and

9

the seeds were also measured with XRD, and those values are summarized in Table Ⅰ with the TDD

10

values of all both samples. Rc of sample A was 0.7 m (concave), which is much smaller compared to

11

the seed A, whose Rc was greater than 30 m. In contrast, Rc was dramatically improved in sample B

12

(>80m) compared to that in seed B (12 m).

13 14

15 16 17

Figure 4. Surface SEM images of (a) sample A and (b) sample B after KOH-NaOH etching for 30 min.

18 19 10


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1

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. TDD and Rc of sample A and sample B before and after HVPE growth on seed A and seed

2

Table

3

B, respectively. -2

Curvature radius (m)

TDD (cm ) Seed

HVPE layer

3

5

3

5

Sample A

10 -10

Sample B

10 -10

7

10 3

5

10 -10

Seed

HVPE layer

> 30

0.7

12

> 80

4 5 6

The generation of cracks and TDs at the epi−sub interface and the increase in Rc after HVPE

7

growth on seed A are very similar to the growth characteristics of AlGaN/GaN.16 In that case, cracks

8

and TDs were generated at the interface due to the large difference in lattice constants. We estimated

9

that a similar lattice mismatch was generated between HVPE layer and seed A, because seed A

10

consists of {101ത1}-plane-growth sectors as depicted in Fig. 5a, which often appeared black color

11

anticipatively due to point defects17 or impurities such as oxygen having strong effects on the lattice

12

parameters18. Given that TDD in the {10 1ത 1}-growth sector is extremely low, as reported

13

previously,19 the blackening is predicted due to the point or impurities defects rather than to TDs. On

14

the other hand, seed B consists of only c-growth sector as depicted in Fig. 5b, which always

15

appeared transparent and colorless as much as HVPE GaN crystal. Therefore, the lattice constants of

16

all growth sectors— c, {101ത2} and {101ത1}— were investigated by measuring reciprocal lattice

17

mapping using synchrotron XRD.

11



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(a)

(b) Sample A

HVPE layer

Sample B

c

HVPE layer

c-Growth sector

seed A (Na-flux GaN)

1

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c

c-Growth sector

seed B (Na-flux GaN) c-Growth sector

c-Growth sector {101ത2}-Growth sector {101ത1}-Growth sector

2

Fig. 5 Schematic drawings of (a) sample A, that is HVPE growth on seed A, which consists of c,

3

{101ത2}, and {101ത 1}-growth sectors, and (b) sample B, that is HVPE growth on seed B, which

4

consists of only c-growth sectors.

5 6

Two kinds of crystals were prepared for measuring the lattice constants. One was a GaN crystal

7

composed of c, {101ത2}, and {101ത 1}-growth sectors (named sample C); and the other was a crystal

8

composed of {101ത2} and {101ത1}-growth sectors (named sample D). The {101ത1}-growth sector in

9

sample D showed a distribution of black-color strength, as shown in the photograph in Fig. 6; the

10

scheme explaining the growth sectors appears at the upper side of the image. Hence, lattice constants

11

were investigated in some points showing different color strengths (indicated as a, b, c, d, and e in

12

Fig. 6) in the case of the sample D measurement. In order to measure the local lattice constants in

13

each growth sector, an X-ray microbeam with a small angular divergence produced with synchrotron

14

radiation,20 which provide high-precision determination of lattice constants, was utilized. The

15

experiments were performed at the Hyogo beamline (BL24XU) of Spring-821. The beam sizes were

16

0.52 × 0.98 µm in sample C, and 35.9 µm × 36.7 µm in sample D.

17

12


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1 2

Figure 6. Cross-sectional photo image of Na-flux-grown GaN crystal (Sample D) grown on a point

3

seed, and the alphabets from “a” to “e” indicate regions where the lattice constants were measured.

4

Schematic drawing of the growth sectors for the cross-sectional image also appears at the upper side

5

of the image.

6 7

Table Ⅰ displays the lattice constants of each measuring point and growth sector. The lattice

8

constants in the c-growth sector were almost the same as those in the {101ത 2}-growth sector; the

9

difference was less than 0.0001 Ⅰ, while those in the {101ത1}-sector were much higher than those in

10

the other sectors; the difference was much more than 0.0001 Ⅰ. Interestingly, the lattice constants at

11

the {101ത1}-sector in sample D showed some distribution; the region with higher black-color strength

12

showed larger lattice constants, indicating that lattice expansion was related to the absorption of

13

visible light. Sequentially, the concentration of oxygen impurity at each growth sector was also

14

measured by secondary ion mass spectrometry (SIMS) analysis, because oxygen was considered the

15

major impurity deriving from the dissolution of the alumina crucible in the Na-flux method, and also 13



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1

had the effect on lattice parameters18. Figure 7 shows the lattice constants as a function of the

2

average oxygen concentration from 1-µm to 5-µm depth. The oxygen concentration of the

3

{101ത1}-growth sector was on the order of 1019 atoms/cm3, which was more than 100 times higher

4

than those of the c and {101ത2} sectors. In both crystals, the lattice constants became larger as the

5

oxygen concentration increased, suggesting that oxygen incorporation caused them to expand. In

6

addition, from Fig. 6 and Fig. 7, oxygen incorporation was also found to make the crystal appearance

7

black in color. Oxygen concentration in HVPE GaN crystal was also investigated, and that was

8

2.2×1016 cm-3. The value was as low as that in c-growth sector in seeds (1.1×1017 cm-3), indicating

9

that lattice constants of c-plane in GaN crystals grown with Na-flux method are almost same as those

10

in HVPE GaN crystals as expected from Fig. 7.

11

14


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. Lattice constants of c-, {101ത2}-, and {101ത1}-growth sectors in sample C grown on a point

1

Table

2

seed (PS) and multi-point seed (MPS), measured using an X-ray microbeam with a small angular

3

divergence produced using synchrotron radiation.

Point

Growth sector

Lattice constant [a-axis] (Ⅰ)

-

c

3.188048

-

3.188044

-

ത 2} {101 ത 1} {101

a

ത 2} {101

3.187975

b

ത 2} {101 ത 1} {101

3.187957

ത 1} {101 ത 1} {101

3.188307

Sample C

Sample D

c d e

3.188248

3.188120 3.188415

4

3.1885

Lattice constant [a-axis] ( Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.1884

Sam ple C c -growth sector Sam ple C {1012}-growth sector Sam ple D {1012}-growth sector Sam ple C {1011}-growth sector Sam ple D {1011}-growth sector

e

3.1883

d

3.1882

c

3.1881

3.1880

3.1879

a b

1017

1018

1019

1020 3

5

Oxygen concentration (atoms/cm )

15



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1

Figure 7. Lattice constants for the a-axis of each growth sector on sample C and sample D, as a

2

function of oxygen concentration. Plots remarked as “a”, “b”, “c”, “d”, and “e” correspond to the

3

measuring points indicated in Fig. 6.

4

Here, we discuss the degradation of sample A’s crystallinity after HVPE growth. Seed A consisted

5

mainly of {101ത1}-planes, whose lattice constants expanded due to oxygen incorporation. The higher

6

concentration of oxygen in the {101ത1} plane than in the c plane is thought to be attributable to the

7

nitrogen-rich surface in the {101ത 1} plane,22 where oxygen atoms form more bonds to the underlying

8

Ga atoms than in other planes allowing for better sticking. The expansion of the lattice constants by

9

oxygen incorporation is thought to be attributable to the increase in the crystal’s free carrier

10

concentration (FCC),23 which causes deformation-potential effect.17 Considering that the growth

11

condition in the Na-flux method often shows nitrogen-poor conditions,24 oxygen atoms are easily

12

incorporated at the nitrogen site, as Wright et al. reported.25 This introduces the activation of oxygen

13

impurity as a donor, resulting in the increase in FCC. High FCC also blackens GaN crystals due to

14

phonon-assisted absorption contributing significantly to the optical losses.26 That finding is in good

15

agreement with the present results.

16

The lattice constants of HVPE GaN crystals are expected to be as large as those of the c-growth

17

sector in Na-flux-grown GaN crystals. Therefore, lattice mismatch between the HVPE GaN layer

18

and seed A, which is about 0.01% as calculated from the lattice constants in table Ⅰ, is larger than

19

that between HVPE and seed B, resulting in the generation of cracks in seed A, and TDs, as well as

20

the increase in Rc after HVPE growth. The wafer showed a curvature of concave, which also

21

indicated that the lattice constants of seed A were larger than those of HVPE crystal. The generation

22

of c-plane cracks in sample A, causing gallium droplet observed in Fig. 2a, was also estimated one of

23

the way to relieve stress due to the lattice mismatch. The mechanism underlying the dramatic

24

improvement in Rc after HVPE growth on seed B is unclear, but it seems that some form of stress

25

for maintaining interstitial lattice constants introduced the effect. The lattice matching between

26

HVPE GaN crystal and Na-flux-grown GaN crystal with the c-growth sectors will be confirmed in a 16


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future study.

2 3 4 5

Summary

6

In this study, we attempted the HVPE growth on two kinds of Na-flux-grown GaN crystals: one

7

composed of {101ത 1}-, {101ത2}-, and c-planes (seed A), and the other composed of only c-planes

8

(seed B). The HVPE layer on seed A showed a degradation of the crystal quality, such as generation

9

of TDs, cracks, and increased Rc, whereas the HVPE layer on seed B showed a perfect propagation

10

of TDs free from the generation of those at the interface, along with improved Rc. The degradation

11

of the crystal quality of the HVPE layer on seed A is attributed to the lattice mismatch between the

12

HVPE layer and seed A, whose lattice constants were expanded in the {101ത1}-growth sectors as a

13

result of the higher concentration of oxygen impurities than were found in the c-growth sectors.

14

TDD values of GaN wafers obtained with this hybrid growth method was as well as those with

15

other methods, such as HVPE on ammonothermal GaN wafers, which are much lower than that of

16

GaN wafers produced with a sole HVPE method. On the other hand, The Rc values in sample B was

17

80 m, which is much higher than those in wafers produced by HVPE on ammonothermal GaN

18

wafers (20 m).27 The higher Rc values in this method can be inferred due to the lower differences in

19

oxygen concentration between the epitaxial layer and the seed. GaN wafers without bowing, having

20

no internal stress, will make it possible to enlarge the wafer diameter without cracking.

21

In conclusion, the differences in oxygen concentrations between a seed and grown crystal for

22

lattice matching, by controlling growth sectors, must be eliminated for homoepitaxial GaN growth

23

when different GaN growth methods are combined. This knowledge is extremely important in the

24

field of the homoepitaxy, and it will be applicable not only to GaN-on-GaN growth, but also to the

25

homoepitaxial growth of other materials for the fabrication of thicker bulk crystals. In addition,

26

HVPE growth on Na-flux-grown GaN crystal which consists entirely of c-planes, is an ideal way to 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

realize the fabrication of large-diameter, low-TDD, fast growing GaN wafers. These wafers will

2

enable the fabrication of GaN devices at mass production line designed only for large-diameter

3

wafers more than 4 inches, and also dramatically reduce the production cost of GaN devices in the

4

future.

5 6

Acknowledgments

7

We gratefully acknowledge funding from the Japan Science and Technology Agency (Project No.

8

J121052565) and from the Ministry of the Environment (Project No. J141057005). The synchrotron

9

radiation experiments were performed at BL 24XU of Spring-8 with approval from Japan Radiation

10

Research Institute (Proposal Nos. 2013B3202 and 2014A3202). We also would like to thank Mr. E.

11

Sawai (Frontier Alliance, LLC) for slicing and polishing specimens.

12

18


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1

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Kimura, H.; Kobayashi, K.; Izumi, K. Jpn. J. Appl. Phys. 2000, 39, L635-L637.

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6

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A.; Turos, A.; Langer, R.; Barski, A. J. Alloys Compd. 1999, 286, 271-275.

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Phys. 2003, 43, L565-L567.

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2015, 432, 49-53.

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For Table of Contents Use Only

2 3

Homoepitaxial HVPE growth on GaN wafers manufactured by the Na-flux method

4

Masayuki Imanishi, Takehiro Yoshida, Toshio Kitamura, Kosuke Murakami, Mamoru Imade, Masashi Yoshimura,

5

Masatomo Shibata Yoshiyuki Tsusaka, Junji Matsui, Yusuke Mori

6

7 8 9

HVPE GaN growth on Na-flux-grown wafers, which is the most promising approach to the fabrication of

10

large-diameter, low-dislocation-density, fast-growing GaN wafers, was attempted for the first time. Lattice

11

matching by controlling growth sectors and oxygen impurities in seed crystals was found to be an important factor

12

for homoepitaxial GaN growth when different GaN growth methods are combined.

13

22


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Figure 1

(a)

(c)

(b)

1 mm/div. c

̅2} plane {101

1 mm/div.

(d)

a m

c plane

̅1} plane {101 c (101̅ 2) (101̅ 1)

c a

c

c m

a

m

43°

62°

200 μm

200 μm

Scale bar: 1 mm

(e)

Scale bar: 1 mm

(f)

1 mm/div.

1 mm/div.

Figure 1. Photo images of as-grown GaN crystals grown with the Na-flux method, which consist of (a) {101̅1}, {101̅ 2}, and c planes (named seed A) and (b) only c planes (named seed B), and bird’s-eye SEM images of the surface of (c) seed A with the scheme describing the crystal-plane orientations at the upper right and the surface of (d) seed B showing an entirely flat c plane. Cross sectional SEM images for determining orientation of planes were in set of Fig 1c and 1d. Photo images after dicing and grinding of the surface of as-grown (e) seed A and (f) seed B. ACS Paragon Plus Environment

1


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Page 24 of 29

Figure 2

(a)

(b)

1 mm/div.

(c)

Scale bar: 5 mm

(d)

Scale bar: 500 μm

Scale bar: 500 μm

Figure 2. Photo images of as-grown GaN crystals after HVPE growth on (a) seed A (named sample A) and (b) seed B (named sample B). White arrows indicate grown-in pits observed on the surfaces of both crystals. The surface morphologies of (c) sample A and (d) sample B, observed by differential interference contrast (DIC) microscopy.


1

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Figure 3

(a)

(b)

m -c

a

1 mm

m a

-c

1 mm

Figure 3. Backside SEM images of (a) sample A, showing some cracks in the seed A, and (b) sample B, showing no cracks in the seed B.


1


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Page 26 of 29

Figure 4

(a)

(b)

Scale bar: 20 μm

Scale bar: 20 μm

Figure 4. Surface SEM images of (a) sample A and (b) sample B after KOH-NaOH etching for 30 min.


1

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Figure 5

(a)

(b) Sample A

HVPE layer

Sample B

c

c-Growth sector

seed A (Na-flux GaN)

HVPE layer

c

c-Growth sector

seed B (Na-flux GaN) c-Growth sector {10 ̅2}-Growth sector {10 ̅1}-Growth sector

c-Growth sector

Fig. 5 Schematic drawings of (a) sample A, that is HVPE growth on seed A, which consists of c, {101̅ 2}, and {101̅ 1}-growth sectors, and (b) sample B, that is HVPE growth on seed B, which consists of only cgrowth sectors.


1


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Figure 6

c a

c-Growth sector

ത 2}-Growth sector {101

m

m

Slice

Point seed

c a

ത 1}-Growth sector {101

c

a c

d

b

m

a

e

2mm

Figure 6. Cross-sectional photo image of Na-flux-grown GaN crystal (Sample D) grown on a point seed, and the alphabets from “a” to “e” indicate regions where the lattice constants were measured. Schematic drawing of the growth sectors for the cross-sectional image also appears at the upper side of the image.


1

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Figure 7

3.1885

Lattice constant [a-axis] (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.1884

c -growth sector Sample C Sample C {1012}-growth sector Sample D {1012}-growth sector Sample C {1011}-growth sector Sample D {1011}-growth sector

e

d

3.1883

3.1882

c

3.1881

3.1880

3.1879

a b

1017

1018

1019

Oxygen concentration (atoms/cm 3 )

1020

Figure 7. Lattice constants for the a-axis of each growth sector on sample C and sample D, as a function of oxygen concentration. Plots remarked as “a”, “b”, “c”, “d”, and “e” correspond to the measuring points indicated in Fig. 6.


1

Homoepitaxial HVPE growth on GaN wafers manufactured by the Na

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