Void Shapes Controlled by Using Interruption-Free Epitaxial Lateral Overgrowth of GaN Films on Patterned SiO2 AlN/Sapphire Template

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Research Article

Void Shapes Controlled by Using Interruption-Free

Epitaxial Lateral Overgrowth of GaN Films on Patterned SiO

₂

AlN/Sapphire Template

Yu-An Chen, Cheng-Huang Kuo, Li-Chuan Chang, and Ji-Pu Wu

Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan

Correspondence should be addressed to Cheng-Huang Kuo; [email protected]

Received 20 February 2014; Revised 26 April 2014; Accepted 2 June 2014; Published 18 June 2014 Academic Editor: Nelson Tansu

Copyright © 2014 Yu-An Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. GaN epitaxial layers with embedded air voids grown on patterned SiO₂AlN/sapphire templates were proposed. Using interruption-free epitaxial lateral overgrowth technology, we realized uninterrupted growth and controlled the shape of embedded air voids. These layers showed improved crystal quality using X-ray diffraction and measurement of etching pits density. Compared with conventional undoped-GaN film, the full width at half-maximum of the GaN (0 0 2) and (1 0 2) peaks decreased from 485 arcsec to 376 arcsec and from 600 arcsec to 322 arcsec, respectively. Transmission electron microscopy results showed that the coalesced GaN growth led to bending threading dislocation. We also proposed a growth model based on results of scanning electron microscopy.

1. Introduction

III-V compound semiconductors of AlN, GaN, and InN are suitable materials for light-emitting diodes (LEDs) because of their wurtzite crystal structures and direct band gap

characteristics [1]. LEDs used for backlighting sources of

liquid crystal displays demand solid-state lighting technology

[2]. However, large lattice mismatches between substrates and

epitaxial layers lead to the formation of threading dislocations (TDs), which decrease the lifetime of diodes and deteriorate

the quality of crystals [3]. Epitaxial films with high

crys-tal qualities are necessary for next-generation applications. Hence, reducing the TD density of epitaxial films is a primary challenge.

Recent studies have proposed several useful growth techniques to improve the crystal quality, such as epitaxial

lateral overgrowth (ELOG) [4–6], pendeo-epitaxy (PE) [7],

maskless PE [8], cantilever epitaxy [9], facet-controlled

epi-taxial lateral overgrowth (FACELO) [10,11], SiNx/GaN buffer

layer [12], abbreviated growth mode [13–15], and freestanding

GaN substrates [16–18]. Patterned sapphire substrate [19] and

embedded air voids method [20, 21] have been developed

to further enhance the light extraction efficiency (LEE) of light-emitting diodes (LEDs). However, embedded air voids method has been widely used. Several kinds of lateral overgrowth techniques have been reported to create air voids,

such as using nanocolumns [22], nanorod [23], PE, or ELOG

technique [24]. Ali et al. also showed that void shapes can

be controlled using different hexagonally patterned maskless GaN templates. The TDs near the voids were bent differ-ently with the various hexagonally patterned maskless GaN

templates [25, 26]. Martinez-Criado et al. used the ELOG

technique to embed air voids into GaN substrate and

recom-mended the stress relaxation and crack suppression [16]. Dai

et al. reported the higher light escaping probability with the chemical etched embedded rhombus-like air voids in

light-emitting structures [27]. In addition, embedded air voids

play a key role in freestanding GaN substrate fabrication. Lin et al. employed the GaN films grown on patterned sapphire substrate with large voids on the top region in the chemical lift-off process and found that the embedded

voids can accelerate the wet etching process [17]. Bohyama

et al. acquired spontaneously separated freestanding GaN substrate by concentrating the compressive stress at the seeds

Volume 2014, Article ID 621789, 8 pages http://dx.doi.org/10.1155/2014/621789

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AlN Sapphire (a) AlN SiO₂ 1.4 𝜇m 2.0 𝜇m (b)

Figure 1: (a) Cross-sectional structure of patterned SiO₂ AlN/sapphire template. (b) Tilted SEM image of patterned SiO₂ AlN/sapphire template.

because of the intentional formation of voids [18].

Neverthe-less, the above-mentioned techniques inevitably suffer from growth interruption and complicated procedures. In ELOG

technique, a 2𝜇m-thick GaN epitaxial layer is deposited on

the substrate, followed by photolithography with dry etching

and photoresist techniques to obtain the templates [4–6].

To satisfy the next-generation application, a high efficiency with a low-cost fabrication method is required. Recently, Lai et al. reported an ex situ AlN buffer layer deposited by sputter, which yielded an interruption-free GaN epitaxy

[28]. To further improve the light extraction efficiency, Sheu

et al. implanted Ar into a sputtered AlN nucleation layer, and their results showed that the GaN-based epitaxial layer grown on implanted regions has lower growth rates than the implantation-free regions, which eventually form the

embedded air voids [29–31].

In this study, we introduced the interruption-free epi-taxial lateral overgrowth (IFELOG) technology, a relatively efficient technique developed to simplify template fabrication while keeping the advantages of ELOG. This technology can

also control void shapes using a patterned SiO₂AlN/sapphire

template and obtain an uninterrupted growth in metal-organic chemical vapor deposition (MOCVD).

2. Experimental Procedure

GaN films used in this study were all prepared by Thomas

Swan(3 × 2󸀠󸀠) MOCVD. A 25 nm thick AlN buffer layer was

initially deposited on a c-plane sapphire substrate by sputter. The AlN plates on the separated sputtering guns were used as the sputtering targets for AlN buffer layer deposition. An

80 nm-thick SiO₂film was deposited on the AlN surface by

plasma-enhanced chemical vapor deposition. The sample was subsequently patterned by photolithography with photoresist

and dry etching processes to form patterned SiO₂microdisks.

High-density plasma was used for SiO₂etching.Figure 1(a)

shows the specification of the patterned SiO₂ AlN/sapphire

template. The pitch and the diameter of the patterned SiO₂

microdisk were 3.4 and 2𝜇m. Figure 1(b) shows the tilted

view of the scanning electron microscope (SEM) image of the

patterned SiO₂AlN/sapphire template.

In this study we controlled void shapes by IFELOG com-prising three growth steps, with each step having a specific function. The first step (Step 1) involved the initial formation of GaN seeds exposed on PVD AlN buffer layer. The second step (Step 2) involved the growth performed only against the c-plane growth, and the final step (Step 3) involved the

coalescence. Trimethylgallium (TMGa) and ammonia (NH₃)

were, respectively, used as gallium and nitrogen sources during growth. The GaN epitaxial layer grown on a patterned

SiO₂AlN/sapphire template with rectangular, triangular, and

pillar voids was labeled as sample-R, sample-T, and sample-P, respectively.

At the onset of GaN growth, the growth temperature and

chamber pressure in Step 1 were set at 1050∘C and 400 torr.

The growth times of sample-R, sample-T, and sample-P in the same step were 1500, 750, and 1500 s, respectively. The growth

temperature and chamber pressure in Step 2 were 1050∘C and

100 torr. Pulsed growth technique was applied to obtain c-plane growth in this step, which is generally used to grow GaN

nanorod arrays [32]. Hence, this technique enhances the

c-plane growth direction. The respective pulsed growth periods of sample-R, sample-T, and sample-P were 60, 60, and 360,

respectively. The flow rates of TMGa and NH₃were 17 sccm

and 3.5 slm; the injection times of TMGa and NH₃were 3 and

5 s. Following the pulsed growth, Step 3 was performed with

growth temperature and chamber pressure set at 1080∘C and

400 torr. Conventional undoped-GaN with neither IFELOG

technology nor patterned SiO₂ AlN/sapphire template was

prepared (i.e., sample-C) for comparative purposes.

The samples were examined by optical microscopy (OM), SEM, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) to discuss the distribution of IFELOG in detail.

3. Results and Discussion

Figure 2 shows the cross-sectional SEM images of GaN epitaxial layers with differently shaped air voids utilizing

IFELOG on the same template.Figure 2(a)shows the

cross-sectional SEM images of sample-R with embedded rectan-gular air voids, each having a width and height of 1.00 and

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1.00 𝜇m 0.10 𝜇m (a) 0.50 𝜇m 0.60 𝜇m (b) 1.12 𝜇m 0.55 𝜇m (c)

Figure 2: Cross-sectional SEM images of (a) sample-R, (b) sample-T, and (c) sample-P.

0.10𝜇m.Figure 2(b)shows the cross-sectional SEM images of sample-T with embedded triangular air voids, each having a

width and height of 0.60 and 0.50𝜇m; the angle of inclination

of the voids was 54∘. Figure 2(c)shows the cross-sectional

SEM images of sample-P with embedded pillar-shaped air

voids, each having a width and height of 0.55 and 1.12𝜇m.

Figure 3(a) shows the growth model of the GaN epitaxial layer with various air voids. At the onset of IFELOG, we obtained different distances between GaN seeds by adjusting

the growth time in Step 1 (Figures 3(a-2) and3(b-2)). We

also controlled the void heights by changing the pulsed

growth periods in Step 2 (Figures3(a-3) and3(c-3)).Figure 4

shows the tilted and cross-sectional SEM images of the GaN epitaxial layers of sample-R after Step-2 growth in the IFELOG technology. The standing GaN seeds confirmed that this technique induced c-plane growth. We also found that the GaN seeds were not able to deposit the entire AlN area during Step-1 growth, which will be discussed in more

detail inFigure 7. Considering that the samples had different

growth time combinations in between steps, we obtained the various diameters and heights of GaN seeds. Ali etal. reported that the diameter of hexagonal holes between GaN affects

void shape control [25]. In other words, the distances between

GaN seeds define the void shape after coalescence. Figures

3(a-4),3(b-4), and3(c-4) show the coalesced growth in Step-3

growth. Narrowing the gap inhibits the source gas molecules from diffusing to the bottom, which eventually forms the

Table 1: Full width at half-maximum values of𝜔 rocking curves measured by XRD. Sample FWHM (arcsecs) (002) (004) (006) (102) (105) Sample-C 485 479 480 600 603 Sample-R 376 362 355 322 384 Sample-T 433 454 420 408 459 Sample-P 416 420 409 360 426

embedded air voids that are responsible for the formation of differently shaped air voids (i.e., sample-R, sample-T, and

sample-P) [33].

The crystal quality of the GaN epitaxial layer was inves-tigated by XRD and AFM. Heying et al. reported the pure edge TD to be insensitive to the symmetric (0 0 l) rocking

curves with𝑙 nonzero and to distort only the (ℎ 𝑘 𝑙) planes

with either ℎ or 𝑘 nonzero. In other words, the decrease

in the FWHM values is regarded as a reduction in TDs [34–36].Table 1shows the respective values of full width at half maximum (FWHM) of the GaN (0 0 2) peak, GaN (0 0 4) peak, GaN (0 0 6) peak, GaN (1 0 2) peak, and GaN (1 0 5) peak for sample-C, sample-R, sample-T, and sample-P, respectively. All of the XRD results showed an improvement in IFELOG, particularly for sample-R, which

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AlN Template (a-1) (a-2) (a-3) (a-4) (a-5) (b-1) (b-2) (b-3) (b-4) (b-5) (c-1) (c-2) (c-3) (c-4) (c-5)

(a) sample-R (b) sample-T (c) sample-P

Step-1 growth Step-2 growth Step-3 growth Sapphire GaN Rectangular air void AlN Sapphire GaN Triangular air void AlN Sapphire GaN Pillar-shaped air void

SiO₂ SiO₂ SiO₂

Figure 3: The proposed growth model of (a) sample-R, (b) sample-T, and (c) sample-P.

GaN SiO₂ (a) GaN Sapphire SiO₂ 1.5 𝜇m (b)

Figure 4: (a) Tilted and (b) cross-sectional SEM images of the sample-R GaN epitaxial layers after Step-1 and Step-2 growth in the IFELOG technology.

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0 2.50 5.00 0 2.50 5.00 (𝜇m) 15.0 7.5 0.0 (nm) (a) 0 2.50 5.00 0 2.50 5.00 (𝜇m) 15.0 7.5 0.0 (nm) (b) 0 2.50 5.00 0 2.50 5.00 (𝜇m) 15.0 7.5 0.0 (nm) (c) 0 2.50 5.00 15.0 7.5 0.0 (nm) 0 2.50 5.00 (𝜇m) (d)

Figure 5: The EPD images over 5× 5 𝜇m2scanning area of (a) sample-C, (b) sample-R, (c) sample-T, and (d) sample-P.

reduced the GaN (1 0 2) from 600 arcsec to 322 arcsec. XRD data showed that IFELOG significantly enhanced the crystal quality due to the lateral growth beside the embedded air voids. The bending dislocations were observed in the TEM images.

A wet etching experiment was conducted in H₃PO₄

solu-tion at 250∘C to determine the etching pit densities (EPDs)

for GaN samples, and the samples were then examined using AFM. After wet etching, numerous hexagonal etching pits were observed on the surface. These etching pits were produced by the threading dislocations propagating to the surface of GaN, which originate from the interface between

GaN and substrate.Figure 5shows the EPD images over a

5 × 5 𝜇m2 scanning area of C, R,

T, and P. The EPDs of C, R,

sample-T, and sample-P were 5.3 × 108, 2.4 × 108, 2.7 × 108, and

2.2× 108/cm2, respectively. These results indicated that the

dislocation densities could be reduced in GaN epilayer using

IFELOG technology, which also corresponded to the XRD result.

TEM was used to analyze the reduction in the dislocation

density. Figure 6 shows the TEM images of the GaN

epi-taxial layer overgrown on the patterned SiO₂ AlN/sapphire

template. The bended TDs were led by the coalesced growth

beside the voids [26], which eventually developed into

stacking faults (inset of Figure 6(c). Recent studies have

reported that TDs can be blocked by SiO₂and stacking faults

mentioned above [37, 38]. Based on the TEM images, the

decrease in FWHM of the GaN peak and EPDs was caused by the significant decrease in TDs of the GaN epitaxial layer through IFELOG.

Figures7(a),7(b), and7(c)show the OM images of

sam-ple-R, sample-T, and sample-P, respectively. The OM images

show that the patterned SiO₂ microdisk was surrounded

by expanded voids. Among all the samples, sample-R and sample-T had the most and least numbers of expanded

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GaN Void Sapphire 1 𝜇m SiO₂ (a) GaN Sapphire Void SiO₂ 1 𝜇m (b) GaN Void Sapphire SiO₂ 1 𝜇m 200 nm (c)

Figure 6: The TEM images of (a) sample-R, (b) sample-T, and (c) sample-P.

(a) (b)

(c)

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GaN Expanded voids SiO₂ (a) Expanded voids GaN Sapphire SiO₂ (b)

Figure 8: (a) The SEM image of sample-R lifted off by diamond cutter. (b) The cross-sectional SEM image of sample-R.

voids. Lift-off template surface and cross-section SEM mea-surements were performed to further clarify the cause of

the expanded voids that surrounded the patterned SiO₂

microdisk. Sample-R was lifted off by a diamond cutter and it was found that the expanded voids surrounded the

patterned SiO₂ microdisk (Figure 8(a)).Figure 8(b) shows

the cross-sectional SEM images prepared to clearly observe

the interface between SiO₂and the GaN epitaxial layer. The

formation of expanded voids was caused by the discontinuous island-type growth characteristic of the GaN seed layer in the beginning. In other words, GaN seeds were not able to deposit the entire AlN area during Step-1 growth, which resulted in

residual vacancies beside the patterned SiO₂microdisk. These

vacancies were converted into expanded voids, which had the similar function with the voids generated from the nanorod

template [23]. Such behavior can increase the lateral growth

and lower the TDs. These expanded voids were suggested to have positive effects on the quality of the GaN epitaxial layer because of the enhancement in XRD data and EPD result of sample-R, which had the most number of expanded voids.

4. Conclusion

We successfully demonstrated an interruption-free epitaxial lateral overgrowth technology by combining sputter AlN buffer layer and pulsed growth method. By adjusting Step-1 growth time and the periods of Step-2 growth, we easily controlled the void shape by the same template. The growth model was proposed to explain the formation of differently shaped air voids based on the SEM results. AFM images show that the epitaxial layer grown by IFELOG technology has lower etching pit densities, thereby increasing the volume of defect-free regions and bending TDs. XRD data suggests that we can decrease the FWHM of the GaN (0 0 2) and (1 0 2) peaks from 485 arcsec to 376 arcsec and from 600 arcsec to 322 arcsec in sample-R. IFELOG technology not only simplified the fabrication of templates, but also greatly enhanced the quality of the GaN epitaxial layer and yielded an uninterrupted growth.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors would like to acknowledge the financial support from National Science Council of Taiwan for their research Grants of NSC 101-2221-E-009-028-MY3 and NSC-3113-P-009-007-CC2.

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