Journal of Crystal Growth 315 (2011) 263–266

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

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

Growth of high-quality InGaN/GaN LED structures on (1 1 1) Si substrateswith internal quantum efficiency exceeding 50%

JaeWon Lee a, Youngjo Tak a, Jun-Youn Kim a,n, Hyun-Gi Hong a, Suhee Chae a, Bokki Min a,Hyungsu Jeong a, Jinwoo Yoo a, Jong-Ryeol Kim b, Youngsoo Park a

a Material & Device Center, Samsung Advanced Institute of Technology, Samsung Electronics, San #14-1, Nongseo-dong, Giheung-gu, Yongin-si, Gyunggi-do 446-712,

Republic of Koreab Department of Optical Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, Republic of Korea

a r t i c l e i n f o

Available online 10 August 2010

Keywords:

A1. Substrates

A1. Defects

A3. Metal organic vapor phase epitaxy

B1. Nitrides

B2. Semiconducting gallium compounds

B3. Light emitting diodes

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.jcrysgro.2010.08.006

esponding author. Tel.: +82 31 280 6825; fax

ail address: junyoun.kim@samsung.com (J.-Y.

a b s t r a c t

GaN-based light-emitting-diodes (LEDs) on (1 1 1) Si substrates with internal quantum efficiency (IQE)

exceeding 50% have been successfully grown by metal organic vapor phase epitaxy (MOVPE). 3.5 mm

thick crack-free GaN epitaxial layers were grown on the Si substrates by the re-growth method on

patterned templates. Series of step-graded AlxGa1�xN epitaxial layers were used as the buffer layers to

compensate thermal tensile stresses produced during the post-growth cooling process as well as to

reduce the density of threading dislocations (TDs) generated due to the lattice mismatches between

III-nitride layers and the silicon substrates. The light-emitting region consisted of 1.8 mm thick n-GaN,

3 periods of InGaN/GaN superlattice, InGaN/GaN multiple quantum wells (MQWs) designed for a peak

wavelength of about 455 nm, an electron blocking layer (EBL), and p-GaN.

The full-widths at half-maximum (FWHM) of (0 0 0 2) and (1 0 �1 2) o-rocking curves of the GaN

epitaxial layers were 410 and 560 arcsec, respectively. Cross-sectional transmission electron

microscopy (TEM) investigation revealed that the propagation of the threading dislocations was

mostly limited to the interface between the last AlxGa1�xN buffer and n-GaN layers. The density of the

threading dislocations induced pits of n-GaN, as estimated by atomic force microscopy (AFM), was

about 5.5�108 cm�2. Temperature dependent photoluminescence (PL) measurements with a relative

intensity integration method were carried out to estimate the internal quantum efficiency (IQE) of the

light-emitting structures grown on Si, which reached up to 55%.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Metal organic vapor phase epitaxy (MOVPE) of GaN on (1 1 1)Si has been regarded as one of the attractive approaches torealize low-cost GaN-based light-emitting-diodes (LEDs) andhigh-electron-mobility-transistors (HEMTs). Whereas typical sub-strates for GaN epitaxy, such as sapphire, silicon carbide, and bulkGaN, are practically limited in size to no more than 4 in.,currently, high-quality low-cost Si substrates are available withdiameters of up to 12 in. In addition, GaN epitaxial growth on Sihas the benefits of a good thermal management, flexibility in chipdesign, compatibility with standard Si processing technologiesand equipment, and the possibility of Si-based opto-electronicintegrated circuits.

The key challenge in growing high-quality GaN epitaxial layerson Si is to overcome the severe lattice (17%) and thermalexpansion (46%) mismatches during the high-temperature

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Kim).

heteroepitaxial growth process. The large lattice mismatchbetween GaN and Si results in the generation of high-density ofthreading dislocations (TDs), on the order of 109

�1010 cm�2,limiting the light-emitting efficiency of the LEDs [1]. Since boththe thermal tensile stresses produced during the post-growthcooling process and the grown-in tensile stresses due to thelattice mismatch between GaN and Si may be easily released bythe cracking of GaN epitaxial layers, the most critical issue is toproperly manage the stress during the whole MOVPE process forthe realization of reliable GaN-based LEDs on Si.

Several approaches were proposed to overcome the inherentdisadvantages of using Si as a substrate for the MOVPE of GaN.Some of the previous studies include the use of AlxGa1�xN-basedstress-compensating buffer structures [2–4], insertion of multiplelow-temperature AlN interlayers [5–7], strain-relieving super-lattice structures [8], sub-monolayer deposition of in-situ SiNx

followed by coalescence growth [3,4], selective-area-growth(SAG) or growth on patterned mesa structures [9–11], and growthon porous Si [12–14].

In this paper, we present the MOVPE of GaN-based light-emitting structures with the use of newly designed buffer layers

J. Lee et al. / Journal of Crystal Growth 315 (2011) 263–266264

on (1 1 1) Si substrates. Re-growth on the patterned templatewith the use of series of AlxGa1�xN buffer layers was effective ingrowing low TD density, crack-free GaN epitaxial layers on Si,suitable for the realization of III-nitride blue LEDs with highinternal quantum efficiency (IQE).

Fig. 2. HRXRD o�2y scan performed on the ‘n-GaN template’ shows the (0 0 0 2)

peaks of the GaN epitaxial layer, step-graded AlxGa1�xN buffer layers, and AlN

layer.

2. Experimental procedure

MOVPE of GaN LEDs on Si was carried out in a Thomas Swanclose coupled showerhead reactor. Two-inch diameter, nominallyon-axis, (1 1 1) oriented Si substrates were used for the hetero-epitaxial growth of III-nitrides. Tri-methyl-gallium (TMGa),tri-methyl-aluminum (TMAl), tri-methyl-indium (TMIn), andammonia (NH3) were used as precursors for Ga, Al, In, and N,respectively. Silane (SiH4) and cyclopentadienyl magnesium(CP2Mg) were used as n-type and p-type doping sources,respectively. Both H2 and N2 were used as carrier gases.

The initial structure consisted of an 80 nm thick AlN nuclea-tion layer, step-graded AlxGa1�xN buffer layers with a totalthickness of 600 nm, and a 200 nm thick undoped GaN (u-GaN)epitaxial layer. The wafers were then patterned as 350 mm�350mm mesas separated by 120 mm wide trenches by induction-coupled-plasma reactive-ion-etching (ICP-RIE) method. Waferswith narrower trench widths of 5 and 10 mm, with the same mesasize, were also prepared to compare the growth uniformity fordifferent trench widths. Then, an 80 nm thick AlN layer, a secondset of 600 nm thick step-graded AlxGa1�xN buffer layers, and a3.5 mm thick GaN epitaxial layer were re-grown on the mesastructure, with the top-half of the GaN doped n-type with Si. TheSi concentration measured by secondary ion mass spectroscopy(SIMS) was 2�1018 cm�3. This structure will be referred to as an‘n-GaN template’ in this work (Fig. 1).

The light-emitting region consisted of a 1.8 mm thick n-GaNlayer, 3 periods InGaN/GaN superlattice structure with a totalthickness of 45 nm, 5 periods InGaN/GaN multiple quantum wells(MQWs) with a well thickness of 3 nm and a barrier thicknessof 8 nm, a 20 nm thick Al0.15Ga0.85N electron blocking layer (EBL),and a 130 nm thick p-type GaN layer.

The crystalline quality of the GaN epitaxial layer was evaluatedby high-resolution X-ray diffraction (HRXRD). The full-widths athalf-maximum (FHWM) of the o-rocking curves were measuredusing both symmetric and asymmetric reflections. Evolutionof the TDs during the heteroepitaxial growth was observed bytransmission electron microscopy (TEM) investigation carried outon a Tecnai F20. Cross-sectional TEM specimens were prepared

Fig. 1. Schematic drawing of the GaN-based

from the center of a selective area grown mesa using a focused-ion-beam (FIB) technique. The densities of TD induced pitsintersecting the ‘n-GaN template’ surface were measured byatomic force microscopy (AFM) in a tapping mode. The opticalproperties of the light-emitting structures were characterized bytemperature and excitation power dependent time-resolvedphotoluminescence (TRPL) measurements. The samples wereexcited with the 380 nm line of a Ti:sapphire laser by varying theexcitation power from 1 mW (32 mW/mm2) to 6 mW (191 mW/mm2). The excited area was limited within one mesa pattern, witha beam diameter of about 200 mm. The relative integrated PLintensities measured at room temperature and low temperaturewere used to estimate the IQE of the LED structures, assuming aunity IQE at low temperature.

3. Results and discussion

Fig. 1 schematically illustrates the epitaxial structure of theGaN-based LED grown on a (1 1 1) Si substrate. It is based on twosets of step-graded AlxGa1�xN buffer layers with re-growth on

epitaxial structure grown on (1 1 1) Si.

J. Lee et al. / Journal of Crystal Growth 315 (2011) 263–266 265

a patterned template. For the re-growth, an 80 nm thick AlN layerwas grown initially to prevent the meltback etching of theexposed Si by Ga [5].

The epitaxial structure re-grown on the patterned templateallowed the growth of thicker crack-free GaN with higher Sidoping, compared to the same structure obtained by theconventional non-patterned growth method, where the crack-free GaN growth was limited to 2.2 mm thick with 1�1018 cm�3

Si doping.Growth nonuniformity was observed for the patterns with

120 mm trench width, resulting in thicker deposition at theperiphery of the mesa. Such nonuniform growth behavior wasreported in selective-area MOVPE of GaN on Si using a SiO2 mask,and it was attributed to the lateral diffusion of chemical specieson the mask region [9]. We observed that the nonuniformity wassuppressed remarkably when the trench width was reduced to

Fig. 3. A cross-sectional TEM image of the GaN-based epitaxial structure grown on

Si, taken under the (1 1 �2 0) two-beam condition, shows the evolution of TDs

during the heteroepitaxial growth.

Fig. 4. (a) The PL spectra at low temperature (blue) and room temperature (red) with 1

IQE of the LED structure reached up to 55% at 191 mW/mm2 excitation power density

referred to the web version of this article.)

less than 10 mm, as reported previously [9]. But it showed only alittle change when the trench width was less than 10 mm.

Fig. 2 shows the HRXRD o�2y scan of the ‘n-GaN template’with a 120 mm trench width, which shows the peaks correspond-ing to the AlN layer, the step-graded AlxGa1�xN stress-compen-sating layers, and the GaN epitaxial layer. The composition of eachAlxGa1�xN layer was calculated as about x¼0.30, 0.50, and 0.75,by assuming Vegard’s law [15]. The AlxGa1�xN composition wascontrolled by varying the gas phase ratio of Al and Ga under thefixed NH3 flow rate, although the actual Al contents in the layersturned out to be lower than calculated. The FWHM of the 3.5 mmthick GaN was 410 and 560 arcsec for the (0 0 0 2) and (1 0 �1 2)reflections, respectively, where the asymmetric reflection isknown to be sensitive to the presence of all types of dislocations.On the other hand, the symmetric reflection is only broadened bythe pure screw type threading dislocations [16,17]. It should benoted that the measured FWHM values were overestimated dueto the mesa patterning and growth nonuniformity, because theHRXRD beam footprint covered multiple mesas and trenches.

The surface morphology and the density of the TD induced pitspropagating to the ‘n-GaN template’ were revealed by AFM in atapping mode. The n-GaN surface has a step-flow morphologywith root-mean-square (RMS) roughness of 0.25 nm in a3�3 mm2 scanned area and 0.86 nm in a 10�10 mm2 scannedarea. The average density of surface pits produced due to theintersecting TDs was about 5.5�108 cm�2.

Evolution of the TDs during the growth of GaN on Si wasobserved by cross-sectional TEM investigation. Fig. 3 shows across-section of the epitaxial structure on a mesa, obtained nearthe center of a mesa with a 120 mm trench width. Under the(1 1 �2 0) two-beam diffraction condition, contrast produced bythe edge- and mixed-type TDs having the Burgers vectorcomponent in the c-plane was revealed. High densities of TDswere present in the step-graded AlxGa1�xN buffer layers. Aremarkable reduction of the TD density was observed at theinterface between the last AlxGa1�xN buffer layer and the GaNepitaxial layer, where most of the TDs terminated. Moreover,some pairs of the propagating TDs in the GaN layers were inclinedfrom the growth axis and annihilated as the growth proceeded,resulting in the high-quality ‘n-GaN template’ suitable for thegrowth of the high-efficiency light-emitting region.

91 mW/mm2 excitation power density show the peak emission at 455 nm. (b) The

. (For interpretation of the references to color in this figure legend, the reader is

J. Lee et al. / Journal of Crystal Growth 315 (2011) 263–266266

Fig. 4 (a) shows the PL spectra obtained at the center of a mesawith a 120 mm trench width at 18 (blue) and 300 K (red) withabout 191 mW/mm2 excitation power density. The peak wave-length of the LED structure was 455 nm. The IQE was estimated asthe ratio of the integrated PL intensity measured at roomtemperature over that at low temperature (I300 K/I18 K), assumingthat radiative recombinations dominate at 18 K so that the IQE isclose to unity at that low temperature [18]. When the excitationpower density of the 380 nm laser was increased from 32 to191 mW/mm2, the IQE reached a maximum of 55% at a 191 mW/mm2 excitation (Fig. 4 (b)). When the Auger non-radiativerecombination term is not dominant, the IQE increases withincrease in excitation power density, or equivalently with carrierdensity, since the radiative recombination term scales with thesquare of the carrier density [18,19]. The radiative and non-radiative recombination lifetimes at 300 K, measured by TRPL,were 9.8 and 12.0 ns, respectively. Such a long non-radiativelifetime of the LED structures indicated that the high-quality GaNepitaxial layer with low TD induced surface pit density of5.5�108 cm�2 effectively reduced the non-radiative carrier loss.Similar ranges of recombination lifetimes were reported in thedouble-heterostructure (DH) GaN LED emitting at 430 nm, wherea peak IQE of 65% was obtained [20]. On the other hand, longerradiative recombination lifetime (5 ns) than non-radiative re-combination lifetime (1 ns) resulted in the IQE less than 17% [21].It should be noted that the light-emitting region has not beenfully optimized yet, and further improvement in the IQE of theLED structure is expected by the structural optimization,suggesting the promising future of the GaN-based LEDs grownon Si.

4. Summary and conclusions

Re-growth on patterned template with the use of series ofstep-graded AlxGa1�xN buffer layers was effective in growinghigh-quality crack-free GaN epitaxial layers on Si, suitable for therealization of high-IQE blue LED structures. 3.5 mm thick crack-free GaN epitaxial layers on Si substrates with (0 0 0 2) and(1 0 �1 2) FWHMs of 410 and 560 arcsec, respectively, and a TDinduced surface pit density of 5.5�108 cm�2 were demonstrated.The IQE of the light-emitting structure grown on this high-quality

GaN on Si template designed with a peak wavelength of 455 nmreached up to 55%.

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