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Journal of Crystal Growth 311 (2009) 3824–3829

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Effect of NH3 flow rate on m-plane GaN growth on m-plane SiC bymetalorganic chemical vapor deposition

Qian Sun a,�, Christopher D. Yerino a, Yu Zhang a, Yong Suk Cho a, Soon-Yong Kwon a, Bo Hyun Kong b,Hyung Koun Cho b, In-Hwan Lee a,c, Jung Han a

a Department of Electrical Engineering, Yale University, New Haven, CT 06520, USAb School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Koreac School of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea

a r t i c l e i n f o

Article history:

Received 2 April 2009

Accepted 13 June 2009

Communicated by R.M. Biefeldimprove the recovery of in situ optical reflectance and the surface morphology, and narrow down the

on-axis (10 1̄0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation

Available online 23 June 2009

PACS:

61.72.Nn

68.55.�A

68.55.�J

81.05.Ea

81.15.Gh

61.05.cp

Keywords:

A1. Morphology

A1. Planar defects

A1. X-ray diffraction

A3. Metalorganic chemical vapor deposition

B2. Semiconducting gallium nitride

B2. Nonpolar

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.06.035

esponding author. Tel.: +1203 535 4637.

ail addresses: qian.sun@yale.edu (Q. Sun), jun

a b s t r a c t

This paper reports a study of the effect of NH3 flow rate on m-plane GaN growth on m-plane SiC with an

AlN buffer layer. It is found that a reduced NH3 flow rate during m-plane GaN growth can greatly

along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends

on the NH3 flow rate, an observation consistent with our recent study of kinetic Wulff plots. The

pronounced broadening of the (10 1̄0) XRC measured along the c-axis is attributed to the limited lateral

coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane

stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron

microscopy.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

III-Nitride semiconductors grown along nonpolar directionsare free of internal electrical field and exhibit a much shorterradiative lifetime than that grown on the c-plane, promising ahigher internal quantum efficiency [1]. There have been someencouraging reports of nonpolar InGaN light-emitting diodes(LED) [2–4] and laser diodes [5–7] through homoepitaxy onexpensive yet very small GaN bulk substrates grown by halidevapor phase epitaxy. Given the limited availability of large-areabulk GaN substrates, growth of nonpolar GaN on foreignsubstrates currently still remains an affordable and criticalapproach to commercialize nonpolar III-Nitride devices withconsistent results. Compared to the heteroepitaxy of c-planeGa- and N-polar GaN [8], nonpolar GaN growth dynamics andmicrostructural evolution have very different characteristics

ll rights reserved.

g.han@yale.edu (J. Han).

[9,10]. Several groups have reported that NH3 flow rate (or V/IIIratio) is one of the most influential parameters for nonpolar a-plane GaN heteroepitaxy, dictating the surface morphology (pitsand striation) and microstructural quality [11–15]. In contrast,only limited work along the same track has been done fornonpolar m-plane GaN (m-GaN). It has been reported thatm-plane LEDs have much better performance than a-plane LEDs[4], which is probably the reason that most of the reportedhomoepitaxial nonpolar devices were grown on the m-plane GaNbulk substrates [2–7]. Therefore, it is worthy to investigate theeffect of V/III ratio on m-GaN growth. In one study, the authorsvaried the Ga beam-equivalent pressure to modify the Ga/N fluxratio in molecular beam epitaxy [16]; however, the m-GaN growthrate was also changed, which may bring in some complexity. Thiswork presents a study of the effect of NH3 flow rate (with a fixedflow rate of Ga source) on the heteroepitaxial nucleationevolution, surface morphology, and microstructural quality ofm-GaN on AlN buffer/SiC. A model combining kinetic Wulff plotswith the coalescence behavior of m-GaN islands is used to explainthe experimental observations.

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Q. Sun et al. / Journal of Crystal Growth 311 (2009) 3824–3829 3825

2. Experiment

m-GaN films were grown in a horizontal metalorganicchemical vapor deposition reactor. Trimethygallium (TMGa),trimethyaluminium (TMAl), and ammonia (NH3) were used asthe precursors for Ga, Al, and N, respectively. After a thermalcleaning of m-plane SiC (m-SiC) substrate, a 220-nm-thick m-plane AlN (m-AlN) buffer layer was deposited at 1150 1C with aV/III ratio of 3400. m-GaN films (�1.1mm thick) were grown onAlN buffer at 1070 1C and 300 mbar with a fixed TMGa flow of230mmol/min and a NH3 flow of 4, 2, and 1 standard liters perminute (slm) for samples A, B, and C, respectively. The V/III ratiosfor the growth of samples A, B, and C were 800, 400 and 200,respectively. Sample D was prepared under the same condition assample C, but with a much shorter GaN growth time for a nominalthickness of 30 nm. Sample surface morphology was studied byNomarski optical microscopy, scanning electron microscopy(SEM) (FEI XL30 field-emission microscope), and atomic forcemicroscopy (AFM) (Digital Instruments Nanoprobe III in tappingmode). A four-axis X-ray diffractometer (Bede D1) was used todetermine the orientation and measure the structural quality ofthe m-GaN films. Transmission electron microscopy (TEM)observations were performed for sample C with a JEOL 3010microscope operating at a 300 kV accelerating voltage.

Fig. 1. (Color online) In situ optical reflectance traces of samples A (a), B (b), and C

(c). The shaded regions correspond to m-AlN and m-GaN growths.

3. Results

The growths of m-GaN/AlN were monitored by in situ opticalreflectance (wavelength ¼ 550 nm). After the thermal cleaning ofSiC substrate, m-AlN buffer was grown in a quasi two-dimensional(2D) mode, implied by its well-oscillating reflectance traces withalmost no decay in the average or damping in the oscillationamplitude (Fig. 1). The as-grown AlN buffer shows mirror-likesurface with a root-mean-square roughness of 1.3 nm for20mm�20mm AFM scan (not shown). However, m-GaN growthreflectance exhibits completely different characteristics with amuch reduced amplitude and a significantly lower average for thefirst oscillation (Fig. 1). The dramatic decay in reflectance impliesa quick-roughening process during the initial growth of m-GaN,which is due to GaN islanding on m-AlN buffer (see the discussionlater). As the m-GaN growth proceeds under various NH3 flows,the reflectance traces presents distinctly different behavior. Forthe m-GaN growth with 4 slm NH3, the reflectance was barelyrecovered (Fig. 1(a)). As the NH3 flow was reduced to 2 slm, thereflectance was slightly recovered for the first three oscillationsand then decayed, shown in Fig. 1(b). With a further reduced NH3

flow of 1 slm, the m-GaN growth reflectance recovered with moresustained oscillations and a much higher average (Fig. 1(c)). It canbe inferred that m-GaN grown under 1 slm NH3 is much smootherthan those grown under higher NH3 flows, which has beenconfirmed by the Nomarski observations. As shown in Fig. 2(a),the surface of m-GaN grown under 4 slm NH3 exhibits a highdensity of striation along the in-plane a-axis [112̄ 0], which is themain source of surface roughness. As the NH3 flow rate decreases,the spatial frequency of undulation across the c-axis [0 0 0 1] isgreatly reduced, giving a much improved surface morphology(Figs. 2(b)–(c)).

Fig. 3 shows the X-ray diffraction 2y/o radial scans in twoorthogonal geometries with the X-ray scattering plane paralleland perpendicular to the direction of striation (the in-plane a-axis[112̄ 0]). Along both the directions, only (10 10) diffraction peaksare observed, confirming that the GaN growth direction is alongthe m-axis. However, it is noted that the m-GaN diffractionintensity in Fig. 3(b) is one order of magnitude lower than that inFig. 3(a), implying much more incoherent scattering of X-ray,

because of the structural imperfections along the in-plane c-axis[0 0 0 1]. The mosaic microstructure in the m-GaN films wascharacterized by the full-width at half-maximum (FWHM) of theon-axis (10 1̄0) X-ray rocking curves (XRCs) measured withthe in-plane a-axis (a-axis broadening) or the c-axis (c-axisbroadening) within the X-ray scattering plane. The c-axisbroadening is always much larger than the a-axis broadening.

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Fig. 2. (Color online) Nomarski optical microscopy images of samples A (a), B (b),

and C (c).

Fig. 3. (Color online) XRD 2y/o scans of m-GaN (10 1̄0) with the in-plane a-axis

(a) and the c-axis (b), within the X-ray scattering plane.

Fig. 4. (Color online) FWHM of m-GaN on-axis (10 1̄0) XRCs measured in two

orthogonal configurations, as a function of NH3 flow rate during GaN growth.

Q. Sun et al. / Journal of Crystal Growth 311 (2009) 3824–38293826

As the NH3 flow decreases, the a-axis broadening of m-GaN getssmaller, while the c-axis broadening gets larger (Fig. 4). It is notedthat the a-axis broadening is reflected by a single narrow peak inthe XRCs, but the XRCs measured along the c-axis contain mainlytwo broad peaks (Figs. 5(a)–(c)). The angular splitting between thedouble peaks increases as the NH3 flow rate is reduced (Fig. 5(d)),which greatly contributes to the overall c-axis broadeningsummarized as the red branch in Fig. 4. In order to study the in-plane mis-alignment (mosaic twist) of the m-GaN films, skewsymmetric XRC measurements were performed through tiltingthe off-axis planes by an inclination angle, w, with respect to theon-axis (10 1̄0). For the inclined (10 1̄ 2) (w ¼ 46.81) and (11 2̄ 0)(w ¼ 30.01), the skew symmetric XRCs measured with the X-rayscattering plane parallel and perpendicular to the striation, give aFWHM of �0.551 and �0.471, respectively, with a minimumfluctuation for samples A, B, and C.

The microstructure of the m-GaN films was further studied byTEM. According to the cross-sectional TEM images (Figs. 6(a) and(f)), the m-GaN film contains a high density (�9�105 cm�1) ofbasal-plane stacking faults (BSFs), while the m-AlN buffer layer

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Fig. 5. (Color online) m-GaN on-axis (10 1̄0) XRCs of samples A (a), B (b), and C (c), with the c-axis within the X-ray scattering plane. The red dash and blue dot lines are

Lorentz fittings for the raw data. (d) The peak splitting of the double peaks in (a)–(c). (For interpretation of the references to colour in this figure legend, the reader is

referred to the web version of this article.)

Q. Sun et al. / Journal of Crystal Growth 311 (2009) 3824–3829 3827

appears much ‘‘cleaner’’, which is likely due to the much higherenergy for the formation of BSFs in AlN than that in GaN [17–19].The onset of the formation of most BSFs occurred at the m-GaN/AlNinterface (Figs. 6(f) and (g)) [20,21]. Meanwhile, selectedarea electron diffraction patterns of m-SiC, m-AlN, and m-GaN(Figs. 6(c)–(e)) further confirmed the epitaxial relationship betweenthe film and the substrate: [101̄0]GaNJ[101̄0]AlNJ[101̄0]SiC,[0 0 01]GaNJ[0 0 01]AlNJ[0 0 01]SiC, and [112̄ 0]GaNJ[112̄ 0]AlN

J[112̄ 0]SiC. The weak streaks along [0 0 01] in Fig. 6(e) are due tothe presence of many BSFs in the m-GaN film [22,23], which are notobserved in Fig. 6(d) for the m-AlN buffer layer. In a separate work,we have demonstrated that the density of BSFs in the m-GaN filmscan be significantly reduced by an insertion of compositionallygraded AlGaN layers between m-GaN and m-AlN [24].

4. Discussion

It has been recognized that both the striation morphology andthe anisotropic linewidths of the on-axis XRCs are related to the

growth evolution of m-GaN on m-AlN buffer [9]. On the as-grownm-AlN buffer surface (observed by AFM), there are many randomshallow grooves (1–2 nm deep) replicating the polishing scratcheson the SiC substrate. During the initial growth of m-GaN on theAlN buffer, GaN nucleates preferentially along the shallow grooveson the AlN buffer surface, where there is a high density of kinksites. Through gas phase and surface diffusion, Ga adatoms areincorporated in a non-uniform manner, forming rod-like m-GaNislands aligned along the pre-existing polishing scratches, as isobserved on the sample D surface (Fig. 7(a)). As these m-GaNislands continue to develop clear facets, including (10 1̄0),(10 1̄1), and (0 0 0 1̄) (Fig. 7(b)), some random nucleation beginsto take place outside the pre-existing scratch lines (Fig. 7(a)). Thisnon-uniform (in space) and incoherent (in time) nucleationbehavior produces islands with varying density, size, and height.As the growth proceeds, the coalescence of adjacent islands alongthe c-axis goes through a concave growth process [25] with anoriginal triangular gap created by the vertical N-face (0 0 0 1̄) ofone island and the inclined (10 1̄1) of its neighbor island. The

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Fig. 6. (a) and (f) Cross-sectional TEM images (g ¼ 11̄0 0) of sample C with the axes labeled in (b); the vertical defects running through the m-GaN layer are identified as

BSFs. Selected area electron diffraction patterns of m-SiC (c), m-AlN (d), and m-GaN (e); (g) cross-sectional HRTEM image of the m-GaN/AlN interface in sample C.

Fig. 7. (a) Perspective view SEM image of sample D (a region having little coverage

of m-GaN). The insets (b) and (c) are the close-up images of two spots, revealing

the faceted islands and their coalescence.

Q. Sun et al. / Journal of Crystal Growth 311 (2009) 3824–38293828

coalescing islands can be very different in height and size(Fig. 7(b)). After the filling up of the triangular gap, a striationalong the in-plane a-axis is often produced and can be observedunder SEM (Figs. 7(b) and (c)). After the deposition of a thickm-GaN layer, the fully covered surface exhibits the typicalstriation morphology shown in Fig. 2.

The m-GaN nucleation and islands coalescence process are alsomanifested in the in situ growth reflectance (Fig. 1). The non-uniform nucleation, the development of m-GaN islands, andislands coalescence result in an overall surface roughening, asrevealed by the decayed reflectance for the first few oscillationsduring the m-GaN initial growth. As the growth proceeds with afull coverage of m-GaN film, the reflectance can be graduallyrecovered, and the final level depends on the surface roughnessdetermined by the overall striation density (Fig. 2).

The final density of surface striation depends on the nuclea-tion, island development, and island coalescence process. Ingeneral, nucleation probability is higher at an increased V/III ratio

[26]. At the wafer edge area (�1 mm) of sample C, a variation ofisland density was observed due to an increased gradient in V/IIIratio from the edge towards the wafer center (Fig. 8(a)).Meanwhile, it is noticed that the m-GaN island shape also variesaccordingly. As shown in Fig. 8(b), the m-GaN islands are verypointed at the very edge (low V/III), but towards the center (highV/III) they become much more elongated along the in-planea-axis, exhibiting flat c-plane (0 0 0 1) sidewalls [27]. This observa-tion of the aspect ratio of m-GaN islands as a function of V/IIIagrees very well with our recent study of kinetic Wulff plots [28].As V/III ratio increases, the lateral growth of m-GaN islands alongthe in-plane a-axis is enhanced while the c-axis growth is sloweddown, giving rise to rod-like island growth. The elongated islandscause densely-spaced undulation along the in-plane c-axis and areresponsible for the pronounced striation morphology of sample A(Fig. 2).

Our previous study [9] has shown that during the coalescencebetween vertical (0 0 0 1̄) and inclined (10 1̄1), the filled-up regionof the triangular gap has more defects (dislocations and/or BSFs)than the original mesa region, and causes inhomogeneous mosaicc-axis broadening as revealed by the shoulder peak in the XRCs(Figs. 5(a)–(c)). It is noted that the FWHM (�11) of the main peakin the XRCs measured along the c-axis is still much larger thanthat of the XRCs measured along the in-plane a-axis. Thisanisotropy of mosaic broadening along the two in-plane ortho-gonal axes has also been observed for sample D (nominally 30 nmthick, with no substantial coalescence along the c-axis). Thisindicates that the onset of the anisotropy in the microstructuralmosaicity occurs at the nucleation of m-GaN on m-AlN bufferlayer, and can be attributed to the formation of BSFs at the m-GaN/AlN interface as confirmed by the cross-sectional TEM observa-tions (Fig. 6). These BSFs significantly reduce the lateral coherencelength along the c-axis [29]. The (10 1̄0) on-axis XRCs measuredalong the c-axis are greatly broadened by a reduced coherencelength, and hence are much broader than the (10 1̄0) XRCsmeasured along the in-plane a-axis. As the NH3 flow rate isreduced during the m-GaN initial growth on AlN buffer, there maybe more BSFs generated at the m-GaN/AlN interface, since zinc-blende GaN nucleation is more favorable under a lower V/IIIcondition [30]. The increase of the BSF density in m-GaN canaccount for the increase of the overall c-axis broadening for the

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Fig. 8. (a) SEM of sample C wafer edge area with discrete m-GaN islands. (b) A

zoom-in image of the bottom-right corner in (a).

Q. Sun et al. / Journal of Crystal Growth 311 (2009) 3824–3829 3829

on-axis (10 1̄0) XRCs as the NH3 flow rate decreases (Fig. 4). Atwo-step m-GaN growth scheme (first high V/III followed by a lowV/III condition) is underway to reduce the BSF density whilemaintaining a relatively smooth surface, which has already beensuccessfully implemented for nonpolar a-plane GaN heteroepi-taxy on the r-plane sapphire [13].

5. Conclusion

In summary, it is found that NH3 flow rates have a profoundeffect on both the surface morphology and the microstructure ofm-GaN films. As the NH3 flow decreases, the surface striationmorphology is significantly suppressed and the on-axis (10 1̄0)XRC measured along the in-plane a-axis becomes much narrower.The pronounced broadening of the (10 1̄0) XRC measured alongthe c-axis is attributed to the influence of the BSFs (originated at

the m-GaN/AlN interface) on the c-axis lateral coherence length ofthe m-GaN mosaic domains.

Acknowledgement

The authors acknowledge the financial support for this workfrom the United States Department of Energy (US DOE) underContract DE-FC26-07NT43227.

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