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Applied Surface Science 362 (2016) 434–440

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

ontrolling the stress of growing GaN on 150-mm Si (111) in anlN/GaN strained layer superlattice

o-Jung Lina,d, Shih-Yung Huangb, Wei-Kai Wangc, Che-Lin Chend, Bu-Chin Chungd,ong-Sing Wuua,c,∗

Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan, ROCDepartment of Industrial Engineering and Management, Da-Yeh University, Changhua 51591, Taiwan, ROCDepartment of Materials Science and Engineering, Da-Yeh University, Changhua 51591, Taiwan, ROCHermes-Epitek Corporation, Hisinchu 30077, Taiwan, ROC

r t i c l e i n f o

rticle history:eceived 2 September 2015eceived in revised form3 November 2015ccepted 24 November 2015vailable online 26 November 2015

a b s t r a c t

For growing a thicker GaN epilayer on a Si substrate, generally, a larger wafer bowing with tensile stresscaused by the mismatch of thermal expansion coefficients between GaN and Si easily generates a crackedsurface during cool down. In this work, wafer bowing was investigated to control stress by changingthe thickness of a GaN layer from 18.6 to 27.8 nm in a 80-paired AlN/GaN strained layer superlattice(SLS) grown on a 150-mm Si (111) substrate. The results indicated that wafer bowing was inverselyproportional to the total thickness of epilayer and the thickness of the GaN layer in the AlN/GaN SLS,

eywords:tressesuperlatticeetalorganic chemical vapor depositionitridesemiconducting III-V materials

since higher compressive stress caused by a thicker GaN layer during SLS growth could compensate forthe tensile stress generated during cool down. After returning to room temperature, the stress of theAlN/GaN SLS was still compressive and strained in the a-axis. This is due to an unintended AlGaN gradinglayer was formed in the AlN/GaN SLS. This AlGaN layer reduced the lattice mismatch between AlN andGaN and efficiently accumulated stress without causing relaxation.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

In recent years, considerable attention has increased on GaN-ased high electron mobility transistors (HEMTs) formed usinglGaN/GaN. Because the GaN-based HEMT shows a lower on-esistance, higher power efficiency, and faster switching speedompared to the conventional silicon power devices, it is expectedo improve power conversion efficiency and contribute to module

iniaturization [1]. In view of mass production, suitable substratesor growing a GaN epilayer are sapphire, silicon carbide, and Si sub-trates. Recently, an increasing number of studies are focusing on aaN epilayer grown on a Si substrate, which has the advantages of

ower cost, a larger wafer size, and favorable thermal conductivity.owever, the mismatch of in-plane lattice constants and thermal

xpansion coefficients (TECs) between GaN and Si are two majorbstacles that must be overcome during epilayer growth. The in-lane lattice mismatch of −16.94% can easily cause defects to affect

∗ Corresponding author at: National Chung Hsing University, Department of Mate-ials Science and Engineering, 145 Xingda Rd., Taichung 40227, Taiwan, ROC.

E-mail address: dsw@dragon.nchu.edu.tw (D.-S. Wuu).

ttp://dx.doi.org/10.1016/j.apsusc.2015.11.226169-4332/© 2015 Elsevier B.V. All rights reserved.

the crystalline quality of the GaN epilayer grown on a Si (111) sub-strate and the performance of the GaN HEMT device. For growing athicker GaN epilayer on a Si substrate, tensile stress caused by a 55%mismatch of the TEC generates a cracked surface during cool downprocess. To solve this, a thinner GaN epilayer, a thicker Si substrate,or a smaller Si substrate can be used. However, these solutions donot have any advantage in device manufacturing and performance.

Various researchers have been working on growing thick GaNepilayers on Si substrates for the applications of high breakdownvoltage devices. Each group has developed its own stress con-trolling technology. In 1999, Ishikawa et al. grew a GaN epilayeron a Si (111) substrate by using AlGaN/AlN transition layer [2].Dadgar et al. demonstrated strain relaxation by inserting a low-temperature AlN layer [3]. Because the relaxed AlN layer hadsmaller lattice constant, compressive stress was induced in the sub-sequently grown GaN layer. The induction of compressive stress isrequired for the growth of thick and crack-free GaN on Si (111)where thermal stress must be compensated for. Feltin et al. found

that the insertion of an AlN/GaN strained layer superlattice (SLS)could reduce tensile stress for avoiding crack formation [4]. Sel-varaj et al. demonstrated that an AlGaN/GaN HEMT on Si (111) withan AlN/GaN SLS exhibited a breakdown voltage as high as 1.4 kV

ace Science 362 (2016) 434–440 435

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Fig. 1. (a) Schematic diagram of growing epi-structure of GaN on 150 mm Si sub-strate with an AlN/GaN SLS. The interface in the AlN/GaN SLS is periodic. (b) STEMimage of the GaN whole structure grown on Si.

Table 1Growth conditions of epi-structure.

Layers Temperature(◦C)*

Pressure(mbar)

NH3

(sccm)V/III

AlGaN barrier layer 1070 50 15,030 3000Undoped GN 1050 200 30,060 150080-paired SLS

GaN 1045 100 15,030 750AlN 1045 100 15,030 7161

Thin GaN layer 1050 200 30,060 1500AlN buffer layer 1100 50 3000 1429

Table 2Structure of samples A, B, C, and D in the SLS.

Sample Growth timeof GaN (s)

Thickness ofGaN (nm)

Thicknessof AlN (nm)

Thickness(nm)

A 16 18.6 4.68 3175

P.-J. Lin et al. / Applied Surf

n 2012 [5]. Ubukata et al. obtained abrupt interfaces in SLS andbserved the strained stress of an AlN/GaN SLS by using asymmet-ical reciprocal-lattice space mapping [6]. Ni et al. determined thatompressive stress in GaN was introduced by the relatively thickerlN layer in an AlN/GaN SLS, which compensated for the tensiletress in the top GaN during cooling [7]. The compressive strain ofhe GaN in the SLS was linearly dependent on the relative thicknessnd partially relaxed AlN in the SLS. The aforementioned literaturesndicate that SLS plays a major role in strain control to grow thehick GaN on a Si (111) substrate.

Although many studies have examined the stress in an AlN/GaNLS, the stress-control mechanism in an AlN/GaN SLS has not yeteen ascertained. Therefore, this paper is aimed to (a) investigate aontrollable stress by adjusting the thickness of the GaN layer in anlN/GaN SLS, and to examine the stress in an AlN/GaN SLS during itsrowth and after cooling; and (b) explore the mechanism for stressccumulation in the AlN/GaN SLS interface without relaxation.

. Experimental procedure

A GaN epilayer with an AlN/GaN SLS was grown on a 150-mm Si111) substrate by using a metal-organic chemical vapor depositionystem produced by Hermes-Epitek. The thickness of the 150-mmi substrate was 675 �m. Trimethylgallium, trimethylaluminum,nd ammonia were used as a precursor for gallium, aluminum, anditrogen, respectively. Hydrogen was used as the carrier gas. Theequences for growing the epi-structure are described as follows:n AlN buffer layer of 150 nm was grown on the Si (111) substrate

o avoid the melt-back etching caused by the reaction between gal-ium and Si. Then, a thin 100-nm GaN was inserted between the AlNuffer layer and SLS. The 100-nm GaN layer was used to smoothhe surface of the AlN buffer layer. The 80-paired SLS, consistingf the AlN and GaN layers, was subsequently grown. Next, a 1-�mndoped GaN layer was grown on the SLS. Finally, an AlGaN barrier

ayer inducing two dimensional electron gases was grown on the 1-m undoped GaN layer. The epi-structure on the Si (111) substrate

s shown in Fig. 1(a). The setting temperature, pressure, ammo-ia flow and V/III ratio of epilayer growths are shown in Table 1. Totudy the controllable stress, samples A, B, C, and D were labeled forhe GaN layers with the thickness of 18.6, 21.6, 24.7, and 27.8 nm,espectively, in the AlN/GaN SLS. The thickness of the AlN layer inhe AlN/GaN SLS was 4.68 nm, and the total thickness of samples, B, C, and D were 3157, 3418, 3589, and 3873 nm, respectively, as

isted in Table 2.An optical microscope was used to explore the surface and the

racks on the GaN epilayer. An X-ray diffraction (XRD) system wasmployed to determine the thickness of a pair in the SLS and therystalline quality of the GaN epilayer. The strain of the GaN epi-ayer was examined by reciprocal space mapping (RSM). An in situurvature monitor and a bowing measurement system were used tonderstand the stress in the AlN/GaN SLS by Stoney’s equation [8]uring its growth and after cooling. Stoney’s equation was basedn five assumptions: 1) The thickness of the substrate and filmhould be uniform, and that of the film less than that of the sub-trate. 2) Deformation is small. 3) The substrate and film shoulde homogeneous, isotropic, and linearly elastic. 4) The stress inhe x- and y-axes should be equal (�X = �Y), and �Z and sheartress should be ignored. 5) The curvature in the x- and y-axeshould be equal, and the twist effect should be ignored. To approacheal stress, researchers have modified Stoney’s formula from 1909.ctually, the AlN/GaN SLS on the Si substrate is nonhomogeneous

nd is not a linearly elastic layer. Moreover, the curvatures in the- and y-axes are unequal (shaped like a potato chip). To quantifyhe stress simply from the in situ curvature monitor during SLSrowth, the AlN/GaN SLS and GaN/AlN/Si were regarded as a film

B 18 21.6 4.68 3418C 20 24.7 4.68 3589D 22 27.8 4.68 3873

and a substrate, respectively. To quantify the stress simply fromthe bowing measurement system after cooling down, the epilayercontaining AlN and GaN was regarded as a homogeneous film onSi substrate. However, the stress of the sub-layer in AlN/GaN SLS

4 ace Science 362 (2016) 434–440

cb

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WtIr(

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3

esst1

Fig. 2. (a) ω-2� scans of the GaN (002) plan. Clear fringes indicate that the interfaceis periodic in the AlN/GaN SLS. The thickness of a pair in the AlN/GaN SLS can be

Fa

36 P.-J. Lin et al. / Applied Surf

ould not be realized. The stress, �, of the bent substrate with aowing or curvature can be calculated using Stoney’s equation.

f hf = Msh2s

6� (1)

hen a layer with the thickness (hf) is grown on a substrate ofhickness (hs), the substrate is usually bent with a curvature (�).n Eq. (1), subscripts f and s denote the epilayer and the substrate,espectively; (M) is the biaxial modulus. The biaxial modulus of Si111) substrate is 179 GPa [9].

The sample D was used to investigate the details of the interfacend the lattice constant, the composition, and the strain mappingf the AlN/GaN SLS by using bright-felid (BF) scanning transmissionlectron microscope (STEM), high angle annular dark field (HAADF)maging and energy-dispersive X-ray spectroscopy, and nano beamlectron diffraction (NBD) technology and, respectively.

. Results and discussion

The BF-STEM image in Fig. 1(b) reveals a distinct interface,specially the interfaces in the 80-paired AlN/GaN SLS. A ω–2�

can of the GaN (002) plane observed through X-ray diffraction ishown in Fig. 2(a). The scan indicates clear fringes, demonstratinghat the interfaces of the AlN/GaN SLS were periodic. Growing a00-nm GaN layer on an AlN buffer layer can smooth the rough

ig. 3. Optical microscope images for investigating cracks generated from wafer edges. (rea of sample B. (e) Image of the edge area of sample C. (f) Image of the edge area of sam

estimated from the period of the fringes. (b) Thickness of a pair in the AlN/GaN SLSas a function of growth time. Thickness is estimated from Fig. 2(a).

AlN buffer layer to obtain flat surface for subsequent SLS growth.

During hetero-epitaxial AlN layer growth, limited lateral growthand mixed-polarity domains typically lead to a poor crystallinequality and rough surface morphology [10,11]. If an AlN/GaN SLS is

a)–(c) Images of edge, middle, and center areas of sample A. (d) Image of the edgeple D.

P.-J. Lin et al. / Applied Surface Science 362 (2016) 434–440 437

Fig. 4. Wafer bowing and stress as a function of epilayer thickness. Stress after cooldp

gG

fttioacaaift3wt−utmistst0SsaaoSctcrme

sttIot

stresses which can be calculated by using Stoney’s equation. The 1-�m-thick undoped GaN was grown on the SLS in part IV. After thisgrowth, tensile stress was generated from the compressive stressduring cool down, shown in part V. For the stress control of AlN/GaN

Fig. 6. (a) Stress–thickness versus thickness of the AlN/GaN SLS. Numbers are

own is calculated using wafer bowing and Stoney’s equation. Stress is inverselyroportional to the total thickness of the epilayer on the Si substrate.

rown on the AlN buffer layer directly without the smooth 100 nmaN layer, the interface would be rough.

The thickness of the pairs in the AlN/GaN SLS can be calculatedrom the period of the fringes, and results are shown in Fig. 2(b). Thehickness of the pairs is linearly dependent on the growth time ofhe GaN layer in the AlN/GaN SLS. The growth rate of the GaN layern the AlN/GaN SLS was 1.535 nm/s. The full width at half-maximumf the rocking curve for GaN (002) and (102) with samples A, B, C,nd D were approximately 680 and 1300 arcsec, indicating that therystalline quality of the GaN epilayer on the Si substrate is accept-ble [6,7]. Sample A, with a total thickness of 3175 nm, has longnd dense cracks in the edge, middle and center areas, as shownn Fig. 3(a)–(c). Fig. 3(d)–(f) shows that there were only short andew cracks around the edge of samples B, C and D, which had largerhicknesses. The total thicknesses of samples B, C and D were 3418,589, and 3873 nm, respectively. The wafer shapes of the samplesere measured using bowing measurement system. Fig. 4 indicates

hat the bowing of samples A, B, C, and D were −110.35, −97.29,89.5, and −77.09 �m, respectively, after cool down. These val-es of wafer bowing are concave. The concave shape attests thathe stress of the epilayer is tensile stress caused by the 55% mis-

atch in the TECs between GaN and Si. The tensile stress resultsn cracking from the edges of the epi-surface. The results in Fig. 4how that a thicker GaN layer in the AlN/GaN SLS, as well as a largeotal thickness, contribute to a smaller wafer bowing and less ten-ile stress. Stoney’s equation was used to estimate the stress ofhe samples. The tensile stresses in samples A, B, C, and D were.133, 0.126, 0.122, and 0.113 GPa, respectively, after cool down.ample D, with thicker GaN layer of 27.8 nm in the AlN/GaN SLS,hows a smaller bowing value of −77.09 �m; this corresponds to

smaller tensile stress of 0.113 GPa compared with samples A, B,nd C. Fig. 3(f) shows that sample D has only few cracks on the edgef the epi-surface. Figs. 3 and 4 show that thinner epilayers on thei substrate obtain higher tensile stresses, creating long and denseracks on the wafer surface. This phenomenon is contradictory tohe common observation of thicker epilayers having long and denseracks. Nevertheless, as discussed in the following paragraphs, theesults for the growth processes characterized by in situ curvatureonitor in conjunction with STEM measurements show supportive

vidence for the observed phenomenon.An in situ curvature monitor was used to understand how wafer

hape changes during AlN/GaN-SLS growth. Fig. 5 shows five parts:he AlN buffer layer, the thin 100-nm GaN layer, the AlN/GaN SLS,he 1-�m-thick undoped GaN, and the cool down are denoted as I,

I, III, IV, and V, respectively. In part I, the AlN buffer layer grownn the Si substrate induced a small tensile stress because the lat-ice constant of AlN is smaller than that of the Si substrate. In

Fig. 5. Data from in situ monitoring of four samples. The AlN buffer layer, the thin100 nm GaN layer, the AlN/GaN SLS, the 1 �m-thick undoped GaN, and cool downare denoted as I, II, III, IV, and V, respectively.

part II, the thin 100-nm GaN layer was used to smooth the inter-face for the following SLS growth. In part III, the AlN/GaN SLSwith 80 pairs continually and linearly accumulated compressivestress with increasing growth time. Because the lattice constant ofGaN was larger than that in AlN, the GaN grown on the AlN couldinduce compressive stress from the lattice difference. The separat-ing slopes in part III depended on the thickness of the GaN layerin the AlN/GaN SLS. The slopes in part III indicated the different

modified from Fig. 5 part III, and based on Stoney’s equation. Slops indicate thecompressive stress during AlN/GaN-SLS growth. (b) Stress versus thickness of theGaN layer. Thickness of the GaN layer in the AlN/GaN SLS can effectively controlstress during AlN/GaN-SLS growth.

4 ace Science 362 (2016) 434–440

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38 P.-J. Lin et al. / Applied Surf

LS, the data in part III was analyzed as shown in Fig. 6. Based ontoney’s equation, the growth rates of 0.34 nm/s and curvatures ofrowing AlN/GaN SLS are considered to satisfy the equation, and theraph of the stress–thickness versus thickness is shown in Fig. 6(a).he slopes in Fig. 6(a) reveal different compressive stresses duringlN/GaN-SLS growth, which are shown in Fig. 6(b). Samples A, B,, and D show compressive stresses of −0.298, −0.377, −0.48, and0.534 GPa, respectively, generated during AlN/GaN-SLS growth.he results are dominated by the thickness of the GaN layer inhe AlN/GaN SLS, which means that sample D, with a thicker GaNayer of 27.8 nm in the AlN/GaN SLS, contributed more compressivetress to the SLS. In contrast to sample D, sample A shows a smallerompressive stress of −0.298 GPa. From the viewpoint of counter-alance, the tendency of compressive stress during AlN/GaN-SLSrowth shown in Fig. 6(b) corresponds to the result of tensile stressfter cool down shown in Fig. 4. Thus, sample D, with its largerompressive stress of −0.534 GPa generated during AlN/GaN-SLSrowth, has a greater ability to compensate for tensile stress causedy cool down, resulting in a smaller bowing of −77.09 �m and aensile stress of 0.113 GPa.

A STEM was used to estimate the in-plane lattice constant of thelN/GaN SLS. Fig. 7(a) shows that the in-plane lattice constants inrea I (GaN), II, and III (AlN) were 0.3163, 0.3139, and 0.3101 nm,

Fig. 7. High-resolution transmission electron microscopy photo a pair in theAlN/GaN SLS. The lattice constant of the a-axis in area I (AlN), II, and III (GaN) isrevealed.

ig. 8. (a) Mapping of lattice constants on the a-axis between AlN/GaN SLS and 1-�m undoped GaN. (b) Mapping of lattice constant on the c-axis in a pair of the AlN/GaNLS.

P.-J. Lin et al. / Applied Surface Science 362 (2016) 434–440 439

F annua e AlN/

recaAdrtubtcStSiSufttircfmidh

aaisigFbaorowaiSo

ig. 9. (a) Distribution of atomic number in a AlN/GaN SLS as shown by a high angletomic number of Ga is light. (b) Energy-dispersive X-ray spectroscopy image of th

espectively. The lattice constants of 0.3163 and 0.3101 nm werestimated from the AlN and GaN layers in the AlN/GaN SLS. As aomparison with the lattice constant of bulk GaN (a0 = 0.318 nm)nd AlN (a0 = 0.311 nm), the strain of the GaN and AlN layers in thelN/GaN SLS was −0.534% and −0.28%, respectively. The resultsemonstrate that the stress of the GaN layer in the AlN/GaN SLSemains markedly compressive after cool down. Fig. 8(a) charac-erize the distribution of lattice constants on the a-axis in the 1-�mndoped GaN and the GaN in the AlN/GaN SLS, by NBD; the scalear is the change in the lattice constant as a percentage in referenceo point x. In Fig. 8(a), the 1-�m undoped GaN is the reference foralculating the distribution of strain in the a-axis. The part of theLS in light blue shows a compressive strain of mostly −1%. Diffrac-ion peaks of the 1-�m undoped GaN and the GaN in the AlN/GaNLS are observed from RSM of GaN (104) shown in Fig. 8(b), whichndicates that the in-plan lattice constant of the GaN in the AlN/GaNLS is relatively smaller (compressive strain) than that of the 1-�mndoped GaN, and the stress of the GaN in the AlN/GaN SLS is mostlyully strain [6]. Fig. 8(c) and (d) characterize the distribution of lat-ice constants on the a-axis and c-axis in the AlN/GaN SLS, by NBD;he scale bar is the change in the lattice constant as a percentagen reference to point x. The GaN layer in the AlN/GaN SLS is theeference for calculating the distribution of strain in the a-axis and-axis. The lattice constant on the a-axis in Fig. 8(c) shows a uni-orm green, which indicates that the strain in the AlN/GaN SLS is

ostly fully strain. The evidences show that the compressive stressn the AlN/GaN SLS exists not only during growth but also after coolown, which explains why sample D, with its thicker GaN epilayer,as fewer cracks and smaller tensile stress (Figs. 3 and 4).

The lattice constant of 0.3129 nm in the area II of Fig. 7 is moder-te. Area II could be the formation of the unintended AlGaN, actings a function of the grading layer created between AlN and GaNn the AlN/GaN SLS. The lattice constant on the c-axis in Fig. 8(d)hows a distinction of layers by using different colors. The blue layerndicates AlN because its lattice constant is the smallest. The AlGaNrading layer is evident by the changing color from blue to green.ig. 9(a) shows the distribution of atomic numbers in AlN/GaN SLSy a HAADF contract image. Because a dark-filed image is gener-ted from crystal diffraction, the grayscale shown in Fig. 9(a) ispposite to that shown in Fig. 1(a) and (b), detailed informationegarding the crystal can be realized, and the unintended AlGaNbviously exists. The grayscale image shows four distinct layers inhich the lower atomic number of aluminum is dark and the high

tomic number of gallium is light [12]. An unintended AlGaN grad-ng layer existed between the AlN and GaN layers in the AlN/GaNLS. The existence of this layer was confirmed through a mappingf the Ga and Al composition as measured using energy-dispersive

lar dark field contract image. The lower atomic number of Al is dark and the higherGaN SLS. Distribution on Al at the interface is evident.

X-ray spectroscopy. The energy-dispersive X-ray spectroscopyimage shown in Fig. 9(b) also confirms that the Al atoms in theAlN layer substitute the Ga atoms in the GaN layer close to the AlNlayer. The mechanism has been examined with GaN/AlN grown onSapphire substrates by Kuchuk et al. and Gogeneau et al. [13,14].This phenomenon is not only thermally active but also related to thestress of GaN/AlN interfaces during AlN growth. Kuckul et al. real-ized that compressive stress in the GaN layer increases the energybarrier for Al-Ga exchange. This AlGaN layer played a critical rolein reducing the mismatch of the AlN and GaN layers and efficientlyaccumulated compressive stress without causing relaxation in theAlN/GaN SLS.

4. Conclusion

In conclusion, the 3873-nm GaN epilayer grown on a 150-mmSi (111) substrate without crack was investigated using 80 pairs ofAlN/GaN in the SLS. The interface of the AlN/GaN SLS was periodic.The compressive stress from −0.298 to −0.534 GPa generated dur-ing the growth of SLS was controllable by adjusting the thickness ofthe GaN layer from 18.6 to 27.8 nm in the AlN/GaN SLS. The thickerlayer of 27.8 nm could compensate for the tensile stress caused bymismatch in TECs between the GaN epilayer and the Si substrateduring cool down, resulting in a flatter bowing of −77.09 �m and asmaller tensile stress of 0.113 GPa. After cool down, a compressivestress remained in the AlN/GaN SLS. The strain state in the AlN/GaNSLS was caused unintended AlGaN. The unintended AlGaN played acritical role in reducing the mismatch between the AlN and GaN lay-ers, and efficiently accumulated stress without causing relaxationin the AlN/GaN SLS.

Acknowledgments

We would like to thank Mr. Chih Sheng Wu of Hermes-EpitekCorporation (Taiwan, R.O.C.) for his encouragement and discussionsfor MOCVD growths. This work was supported by the Ministry ofScience and Technology (Taiwan, R.O.C.) under contract nos. 101-2221-E-005-023-MY3 and 104-2622-E-005-005-CC2.

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