Cite this: CrystEngComm, 2013, 15,3372

Effects of AlN interlayer on growth of GaN-based LEDon patterned silicon substrate

Received 19th December 2012,Accepted 16th February 2013

DOI: 10.1039/c3ce27059a

www.rsc.org/crystengcomm

Junlin Liu,* Jianli zhang, Qinghua Mao, Xiaoming Wu and Fengyi Jiang

GaN-based light emitting diodes (LEDs) were grown on 1 mm 6 1 mm patterned 2-inch and 6-inch Si

(111) substrates by metal–organic vapour phase epitaxy (MOVPE). AlN interlayers with different

thicknesses were introduced between the composition-graded AlGaN buffer layer and the GaN seed

layer in different LED structures. The crystalline quality, wavelength uniformity and crack density of the

2-inch wafer were improved by increasing the AlN interlayer thickness. With a 30 nm AlN interlayer, a

crack-free, smooth and reflective 6-inch LED wafer was grown, the full width at half maximum (FWHM) of

the XRD rocking curves of GaN (002) and GaN (102) planes were 384 and 432 arcsec, respectively. The

standard deviation of thickness and dominant wavelength were 0.03 mm (average thickness was 3.84 mm)

and 1.52 nm (average dominant wavelength was 457.9 nm), respectively. The AlN interlayer can change

the growth mode of the GaN seed layer and subsequent n-GaN, which changes the density of dislocation

and the residual tensile stress of the GaN film. A smaller residual tensile stress in the GaN film can help to

reduce bowing of the wafer, improve the wavelength uniformity, and suppress the generation of cracks.

1. Introduction

In spite of the use of GaN-based light emitting diodes (LEDs)being now widespread in applications such as display panels,backlighting, traffic lights, etc., several challenges remain ifLEDs are to make a significant penetration into the generallighting market. The most important one is the cost. HavingGaN-based LEDs on silicon is an attractive way to lower thecost because the silicon substrate is available in large sizes, upto 300 mm diameters, with low cost and high quality(compared to sapphire and SiC substrates). So, more andmore LED companies and research institutes are participatingin the research of LEDs on silicon. However, the majorchallenge is the high mismatch in the lattice (17%) andthermal expansion coefficients (46%) between GaN and Si,which induces significant tensile stress in the GaN epilayer,which in turn often leads to wafer bowing and/or cracking.1 Inorder to control the tensile stress and associated cracking andbowing, several methods have been proposed, such as low-temperature AlN interlayer technology,2 graded AlGaN buffertechnology,1,3,4 patterned Si substrate technology5–9 and othertechnologies.10,11 In our opinion, the patterned Si substratetechnology is a very effective method to control the crackingand bowing of the wafer, especially for a large size wafer. Thesize of the LED diodes can vary within the range from 0.1 mm6 0.1 mm to 2 mm 6 2 mm by changing the pattern sizes on

the Si substrate, which can easily fulfill the demand of most ofthe LEDs applications. In this paper, we report one method toimprove the quality of GaN-based LEDs on a patterned siliconsubstrate. The tensile stress of the LED film is decreaseddramatically, the crystalline quality and wavelength uniformityare improved by using this method.

2. Experimental

The GaN structures were grown on patterned 2-inch and 6-inchSi (111) substrates by metal organic vapour phase epitaxy(MOVPE) in a 7 6 299 Thomas Swan CCS reactor using a 7 6299or 1 6 699 susceptor. Trimethylgallium (TMG), trimethyla-luminium (TMA) and trimethylindium (TMI) were used asgroup-III precursors, and ammonia was used as the nitrogensource. Silane (SiH4) and cyclopentadienyl magnesium(Cp2Mg) were used as the sources of the n-type and p-typedopants, respectively. The surface of the Si (111) substrateswere patterned in a size of 1 mm 6 1 mm by 15 mm-widthtrenches. The buffer layer was composition-graded AlGaN,which consisted of 80 nm thick AlN, 120 nm Al0.7Ga0.3N and 80nm of Al0.3Ga0.7N, sequentially. Three templates were preparedto study the nucleation of the GaN seed layer on an AlGaNbuffer with different thicknesses of an AlN interlayer, labeledas T1, T2 and T3, as shown in Fig. 1. The thickness of the AlNinterlayer between the AlGaN buffer layer and the GaN seedlayer for templates T1, T2 and T3 was 0 nm, 10 nm and 30 nm,respectively. The three LED structures consisted of 2.8 mmn-GaN (Si doped concentration is 2 6 1018 cm23), seven-

National Engineering Technology Research Center for LED on Si Substrate, Nanchang

University, Nanchang 330047, People’s Republic of China.

E-mail: liujunlin@ncu.edu.cn; Fax: +86 79188304441; Tel: +86 79188317293

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periods InGaN/GaN multiple quantum well (MQW) and 130nm p-GaN were then grown on templates T1, T2 and T3, whichwere labeled as L1, L2 and L3, respectively. The growthtemperatures of the composition-graded AlGaN buffer, AlNinterlayer, GaN seed layer and n-GaN were all 1030 uC. Thegrowth pressures of the composition-graded AlGaN buffer, AlNinterlayer, GaN seed layer and n-GaN were all 133 mbar. The V/III ratio of the GaN seed layer growth was 500. The samples T1,T2, T3, L1, L2 and L3 were all grown on 1 mm 6 1 mmpatterned 2-inch Si (111) substrates (430 mm thickness).Additionally, a LED structure L4 with the same structure asL3 was grown on a 1 mm 6 1 mm patterned 6-inch Si (111)substrate (1 mm thickness). The samples were characterisedby X-ray diffraction (XRD), atomic force microscopy (AFM),scanning electron microscopy (SEM), photoluminescence (PL)and optical microscopy (OM).

3. Results and discussion

Fig. 2 shows the full width at half maximum (FWHM) of theXRD rocking curves of the GaN (002) and GaN (102) planes of

three LED structures, L1, L2 and L3. It can be seen that theFWHM of the XRD rocking curves of the GaN (002) and GaN(102) planes decrease obviously with the increasing thicknessof the AlN interlayer. In other words, the crystalline quality ofthe LED is improved with increasing AlN interlayer thickness.

Fig. 3 shows the radial distribution of the dominantwavelength of the three LED wafers. It is shown that thedominant wavelength increases gradually from the center tothe edge of the wafer. With increasing AlN interlayer thickness,the difference between the highest and the lowest dominantwavelength becomes smaller. The differences are 19.4 nm,11.0 nm and 3.7 nm for samples L1 (without AlN interlayer), L2(with a 10 nm AlN interlayer) and L3 (with a 30 nm AlNinterlayer), respectively. The growth temperature of the MQWwas about 300 uC lower than that of n-GaN, and the thermalexpansion coefficient of GaN (5.6 6 1026 K21) is higher thanthat of Si (2.6 6 1026 K21). Therefore, the wafer becameconcave-bowed during the growth of MQW due to thermalstressing between the GaN layer and Si substrate. Then, thetemperature at the edge became lower than in the centre, thusthe wavelength at the edge was longer than that in the center.During the growth of GaN on the AlN interlayer, compressivestress is induced. The compressive stress increases with thedecreasing dislocations, and this compressive stress cancompensate the tensile stress during cooling, and then reducethe residual tensile and bowing of the wafer. So, thewavelength uniformity becomes better from sample L1 to L3.

Fig. 4 shows the thickness and dominant wavelengthmapping images and a photograph of the sample L4 on a6-inch Si substrate. The surface of L4 is smooth, reflective andcrack-free. The FWHM of XRD rocking curves of GaN (002) andGaN (102) planes are 384 and 432 seconds, respectively. Thestandard deviation of thickness and dominant wavelength are0.03 mm (average thickness is 3.84 mm) and 1.52 nm (averagedominant wavelength is 457.9 nm), respectively. This revealsthat the 30 nm AlN interlayer is effective to reduce bowing andimprove wavelength uniformity for both a 2-inch and 6-inchwafer.

Fig. 5 shows the optical microscopic images of samples L1,L2 and L3 recorded 180 days after the growth. One thing

Fig. 2 Dependence of FWHM on the AlN interlayer thickness of the three LEDstructures L1, L2 and L3.

Fig. 3 The radial distribution of dominant wavelength of three LED wafers.

Fig. 1 A schematic illustration of three templates; T1 without an AlN interlayer,T2 with a 10 nm AlN interlayer, T3 with a 30 nm AlN interlayer.

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should be noted, that the three samples were crack-free justafter the growth. It is shown in Fig. 4 that the samples L1 andL2 have many cracks and sample L3 is crack-free after aging(stored for 180 days without external stress), and the crackdensity of sample L1 is much larger than that of sample L2. Ingeneral, the GaN on Si is suffering from residual tensile stress,which relaxes slowly by generating cracks. The larger theresidual tensile stress, the higher the crack density. If theresidual tensile stress was very small, the crack wouldn’t begenerated. So, we conclude that the residual tensile stress isremarkably decreased with increasing AlN interlayer thick-

ness. With a 30 nm AlN interlayer, the LED film was crack-freeand stable after 180 days.

In order to study the influence of the AlN interlayer on theLED film, we investigated the three templates by SEM, asshown in Fig. 6. This reveals the GaN seed layer morphologiesof the templates with different AlN interlayers, the surface ofthe template consisted of GaN seeds and voids. Withincreasing AlN interlayer thickness, the void density increasesand the GaN seed size decreases correspondingly, and theshape of the GaN seeds change from a connected layer toislands. The epitaxial growth modes are divided simply into

Fig. 4 (a) Thickness map, (b) PL emission wavelength map and (c) a photograph of the sample L4 on a 6-inch Si substrate.

Fig. 5 The optical microscopic images of three samples after storing for 180 days (a) L1, (b) L2 and (c) L3.

Fig. 6 The SEM images of three samples (a) T1, (b) T2 and (c) T3.

3374 | CrystEngComm, 2013, 15, 3372–3376 This journal is � The Royal Society of Chemistry 2013

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three types,12 layer by layer mode (Frank–Van der Merwemode), islands mode (Volmer–Weber mode) and layer andislands mode (Stranski–Krastanow mode). The misfit betweenthe epitaxy layer and the substrate largely affects the growthmode. The epitaxy layer tends to grow in islands mode whenthe misfit is large, and it will tend to grow in layer by layermode when the misfit is small.12

The epitaxy layer is the GaN seed layer, and the substratesare Al0.3Ga0.7N, a 10 nm AlN interlayer and a 30 nm AlNinterlayer for samples T1, T2 and T3, respectively.

The theoretical in-plane lattice constant of the AlNinterlayer can be calculated by eqn (1):13

a = a0 + hc/h?(as 2 a0) (1)

where a is the in-plane lattice constant of the AlN interlayer, a0

is the lattice constant of unstrained AlN, hc is the coherentgrowth critical thickness of AlN on Al0.3Ga0.7N, h is thethickness of the AlN interlayer and as is the lattice constant ofAl0.3Ga0.7N.

The lattice constant of AlxGa(12x)N is given by eqn (2):

aAlxGa(12x)N = x?aAlN + (1 2 x)?aGaN (2)

where aAlN is the lattice constant of AlN (aAlN = 0.3112 nm) andaGaN is the lattice constant of GaN (aGaN = 0. 3189 nm). So,aAl0.3Ga0.7N is equal to 0.3166 nm. According to the methodmentioned in ref. 13, the calculated coherent growth criticalthickness hc of AlN on Al0.3Ga0.7N is 9.88 nm.

According to the calculation, the misfit between the epitaxylayer and the substrate is 0.73%, 0.76% and 1.92% for samplesT1, T2 and T3, respectively. For sample T3, the misfit betweenthe epitaxy layer and the substrate is 1.92%, which is largeenough to make the GaN seed layer grow in islands mode, asshown in Fig. 6(c). For samples T1 and T2, the misfit betweenthe epitaxy layer and the substrate is smaller, so the GaN seedlayer in T2 preferentially grows in layer and islands mode. Incontrast, the growth mode of sample T1 was very close to layerby layer mode, as shown in Fig. 6(b) and (a).

The LED structures L1, L2 and L3 had differences duringthe beginning growth stage of the n-GaN layer. Sample L1 grewmainly according to step-flow mode and had less epitaxiallateral overgrown (ELO) on the site of the voids. However,sample L3 grew mainly according to ELO on the large numberof void sites in the beginning. After the islands coalescedgradually into a flat plane, it then grew according to step-flowmode. Sample L2 was similar to sample L1, it just had moreELO than sample L1. The AFM surface morphology of samplesL1, L2 and L3 are flat and step-flow-like, and the roughness of2 mm 6 2 mm were 0.228 nm, 0.214 nm and 0.197 nm,respectively. During the ELO, the density of dislocationdecreased.14,15 So, the dislocation density of sample L3 wasthe smallest, and that of sample L1 was the highest. Sample L2is between sample L1 and L3. The FWHM of XRD rockingcurves of the GaN (002) and GaN (102) planes decrease fromsample L1 to L3, as shown in Fig. 2. During the growth of GaNon the AlN interlayer, compressive stress is induced. The

compressive stress increases with the decreasing dislocations,and this compressive stress can compensate the tensile stressduring cooling, and then reduce the residual tensile andbowing of the wafer. So, the wavelength uniformity becomesbetter from sample L1 to L3, as shown in Fig. 3. Thewavelength uniformity of the 6-inch wafer was also good.Besides, the crack density decreases from sample L1 to L3, asshown in Fig. 5.

4. Conclusions

The influence of AlN interlayer thickness on the quality ofGaN-based LEDs on silicon substrate was investigated. Withincreasing AlN interlayer thickness, the crystalline quality,wavelength uniformity and crack density of the 2-inch waferwere improved. The 6-inch wafer with a 30 nm AlN interlayerhad good thickness and wavelength uniformity, and thestandard deviation of thickness and dominant wavelengthwere 0.03 mm and 1.52 nm, respectively. The reason for suchresults is as follows: the growth mode of the GaN seed layerchanged from near layer by layer mode to islands mode, andthe subsequent growth mode of the beginning stage of n-GaNgrowth changed from step-flow oriented to ELO oriented withincreasing AlN interlayer thickness, which decreases thedensity of dislocation. During the growth of GaN on the AlNinterlayer, the compressive stress is induced. The compressivestress increases with the decreasing dislocations. Thiscompressive stress can compensate the tensile stress duringcooling, and then reduce the residual tensile stress andbowing of wafer. A smaller residual tensile stress in the GaNfilm can reduce bowing of the wafer, improve the wavelengthuniformity, and suppress the generation of cracks.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 51072076) and by the National KeyTechnology Research and Development Program of China (No.2011BAE32B01) and by the National High TechnologyResearch and Development program of China (No.2011AA03A101) and by the Special Project on Science andTechnology of Jiangxi, China (No. 20114ABF06103).

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