Journal of Crystal Growth 204 (1999) 270}274

Single-crystal GaN pyramids grown on (1 1 1)Si substratesby selective lateral overgrowth

Wei Yang!, Scott A. McPherson!, Zhigang Mao", Stuart McKernan",C. Barry Carter",*

!Honeywell Technology Center, 12001 State Hwy. 55, Plymouth, MN 55441, USA"Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455-0132, USA

Received 25 January 1999; accepted 9 April 1999Communicated by T.F. Kuech

Abstract

Single-crystalline GaN/AlN layers have been grown on (1 1 1)Si substrates and subsequently used as the seeding layerfor selective lateral overgrowth on a patterned Si

3N

4mask. GaN pyramids are formed during the lateral overgrowth.

They are epitactically oriented with respect to the Si substrate with the orientation relationships [1 1 21 0]G!N

E[11 1 0]S*

and (0 0 0 1)G!N

E(1 1 1)S*. The pyramids were characterized by transmission electron microscopy and were found to have

a reduced defect density compared with continuous layers of GaN on Si and very few defects in the upper portion. Thethreading dislocations that originated from the GaN/AlN seeding layer eventually bend through 903 and emerge at theinclined surface. Many of the threading dislocations are actually half-loops which are self-terminating. The results of thisstudy demonstrate that GaN selective lateral overgrowth can be performed on Si substrates and show that the same GaNcrystalline quality can be produced on Si as on sapphire and SiC substrates. ( 1999 Elsevier Science B.V. All rightsreserved.

1. Introduction

Selective lateral overgrowth can be an e!ectivetechnique for defect reduction in GaN hetero-epitaxy [1}8]. Laterally grown GaN on sapphiresubstrates has recently been used successfully forlong-lifetime CW InGaN laser diodes [1,2]. Defectreduction has been observed in GaN crystalsgrown from striped windows and from pinholes[3,4]. Of particularinterest to this study is the lat-

*Corresponding author. Tel.: #1-612-625-8805; fax: #1-612-626-7246.

E-mail address: carter@cems.umn.edu (C.B. Carter)

eral overgrowth using small pinholes [4}7]; GaNhexagonal pyramids are formed and laterallygrown to sizes much larger than the seeding area.The laterally overgrown portions, which constitutethe majority of the deposition, have no direct con-tact with the substrate, thus it might be expectedthat their quality becomes less dependent on thesubstrate selection.

So far, most reported lateral growth has beenconducted on sapphire and SiC substrates. Theselection of these substrates is partly because oftheir availability with pregrown single-crystallineGaN layers which are then used as the `seedinglayera under the mask. This seeding layer is thoughtto be necessary for the initiation of single-crystal

0022-0248/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 2 0 5 - 5

growth. Si substrates would be of great interestbecause of their low cost, large size, and potentialfor the integration of GaN-based optoelectronicdevices and Si-based electronics. Additionally, inthe fabrication of laser diodes, it is much easier toremove the Si substrate than the sapphire one forthe backside electrical and thermal contacts. Thesame advantage is given by GaN-substrate fabrica-tion on a sacri"cial substrate; a Si sacri"cial sub-strate would be much easier to remove thana sapphire one.

AlN and GaN single-crystalline "lms have beengrown on Si substrates previously [9}14]. Severalintermediate layers including 3C-SiC [11], AlN[12,14], and c-Al

2O

3[13] have been found to be

e!ective in initiating single-crystal wurtzite GaNgrowth. Watanabe et al. [12] found that directgrowth of GaN on (1 1 1)Si substrates resulted inpolycrystalline depositions, and that a single-crys-tal, AlN intermediate layer grown at high-temper-atures (high-¹ bu!er layers) is essential for thesubsequent growth of a single-crystal GaN layer[12]. The observations reported here support theconclusions of other researchers. However, onlylimited microstructural information has been re-ported for these materials; GaN grown on Si sub-strates does not appear to be used at present inhigh-performance GaN devices which suggeststhat the inferior crystal quality (higher dislocationdensities) compared to those grown on sapphireand SiC substrates persists. The lower quality ofGaN on Si may be attributed to the largerdi!erences in lattice constants between GaNand Si. In the lateral overgrowth scheme,however, the dislocations in the GaN seedinglayer are not so critical because they only a!ecta small volume of the lateral overgrown mate-rial. The present study demonstrates that, usinglateral overgrowth, the GaN quality can be im-proved to the same extent as GaN grown on sap-phire and SiC substrates using the same method.GaN pyramids have been grown on GaN/AlNseeding layers which in turn had been grown on(1 1 1)Si substrates. Microstructural characteriza-tion of GaN pyramids and the underlying layerswas performed using scanning electron microscopy(SEM) and transmission electron microscopy(TEM).

2. Experimental procedure

The sample growth was conducted in a low-pressure metal-organic chemical vapor deposition(MOCVD) system with a vertical reactor. A 2A(1 1 1)Si wafer was etched in HF : H

2O, rinsed and

dried immediately before loading. The system wasevacuated before the pressure was regulated at 10Torr with a constant H

2#ow. The susceptor was

inductively heated to 11503C and the Si wafer wasbaked at this temperature for 10 min. Deposition ofthe AlN bu!er layer was initiated by #owingtriethylaluminum (TEA) and ammonia (NH

3) into

the reactor at 1 lmol/min and 1.2 slm while thesusceptor temperature was maintained at 11503C.The AlN bu!er layer was grown for 20 min, result-ing in a thickness of 100 nm. The GaN layer wassubsequently grown with a triethylgallium (TEG)#ow of 6.5 lmol/min. After the TEA #ow was stop-ped, the GaN layer was grown for 30 min, resultingin a thickness of about 150 nm. The as-grownGaN/AlN "lm was smooth with a mirror surface.For the selective growth, a 100 nm Si

3N

4masking

layer was deposited on the wafer by plasma-en-hanced chemical vapor deposition (PECVD). Ar-rays of openings of 5 lm diameter and 20 lmcenter-to-center pitch in close-packed format werecreated over the whole wafer by photolithographyand a reactive-ion etch (RIE). The wafer was re-loaded into the MOCVD system for the lateralovergrowth. The system pressure was set at 76 Torrand the ammonia #ow was at 1.8 slm. The growthstarted as soon as the susceptor temperature reach-ed 11503C. The total growth time was 3 h. TheTEG #ow was 1.9 lmol/min for the "rst hour and5.3 lmol/min for the remaining 2 h. The smallerTEG #ow for the initial growth was used to avoidnucleation on the mask.

3. Experimental results and discussion

Selective growth was achieved on all wafers witha patterned Si

3N

4mask; the masks were free from

cracks and GaN nucleation did not occur on themask. Although it would be desirable to grow theAlN bu!er layers at low temperature (800}10503C),such bu!er layers resulted in GaN growth through

W. Yang et al. / Journal of Crystal Growth 204 (1999) 270}274 271

Fig. 1. SEM image of the GaN pyramids grown on a (1 1 1)Sisubstrate by selective lateral overgrowth.

the windows which was polycrystalline with noapparent preferred orientations with respect to theSi substrates; pyramid formation was not observed.On the wafer with the 11503C AlN bu!er layer,pyramid growth was observed over the entire wa-fer. An SEM view of the GaN pyramids in shown inFig. 1, and has a similar appearance to pyramidsgrown on sapphire or SiC substrates. Some (1 11 0 1)facets exhibit faster growth rate than others, pos-sibly due to the e!ect of the side walls of adjacentpyramids. The base diameter of the pyramids isabout 16 lm, substantially larger than the 5 lmopenings, thus the crystals had grown laterally overthe mask. The shape of the openings does not a!ectthe pyramid formation and orientations. All thehexagonal pyramids exhibit identical crystallo-graphic orientations. These features of the GaNpyramids indicate that the GaN/AlN seeding layerwith the high temperature AlN bu!er layer success-fully initiated single-crystal wurtzite GaN growth.These observations are in good agreement withthose of Watanabe et al. [12].

TEM study shows that the GaN pyramid, theGaN "lm, and the AlN bu!er layer are monocrys-talline with an epitactic orientation relationshipwith respect to the Si substrate given by[1 1 21 0]

G!NE[1 1 21 0]

A-NE[11 1 0]

S*and (0 0 0 1)

GaNE(0 0 0 1)AlNE(1 1 1)Si, which is consistentwith the results of Watanabe et al. [12]. A selected-area di!raction (SAD) pattern from the GaN/AlN

Fig. 2. (a) The SAD pattern from the GaN layer and the AlNbu!er layer, the common zone axis is [1 1 21 0]. The streaks ofthe re#ections in the "gure are due to the small di!erence in thelattice constants between GaN and AlN.

Fig. 3. A multi-beam bright-"eld image of the interfacial regionbetween the pyramid and the substrate.

layers and the pyramid is shown in Fig. 2 whichillustrates the excellent crystal quality and epitacticalignment of the layers. Fig. 3 is a cross-sectionalTEM image of the GaN/AlN layers and the AlN/Siinterfacial region. The GaN/AlN layers have anestimated dislocation density of &1012 cm~2. Thethreading dislocations are observed to propagatethrough both the AlN and GaN layers.

Fig. 4 is a dark-"eld (DF) TEM image of thecenter-cut cross-sectional specimen of the pyramid.The upper third of the pyramid (above 7 lm) isvirtually defect free. The highest defect density wasobserved in a cone-shaped region above the initialseeding area (the mask opening), which is similar to

272 W. Yang et al. / Journal of Crystal Growth 204 (1999) 270}274

Fig. 4. Dark-"eld image at a low magni"cation taken near the[1 1 21 0] GaN zone axis with u"0 0 0 2 revealing the disloca-tion distribution in the whole GaN pyramid.

the observation of Zheleva et al. [4]. This internalregion is not sharply de"ned but is approximatelypyramidal in shape. Horizontally propagating dis-locations are present in the lower portion of thepyramid; these extend to the (1 11 0 1) surfaces ofthe pyramid. These dislocations result when thethreading dislocations originated from the seedinglayer abruptly bend through 903, as is more clearlyshown in Fig. 5. The 903 bends occur close to thesurface of the internal pyramid which encloses thehigh-defect-density region. This observation sug-gests that the initial GaN growth and defectpropagation took place layer-by-layer in the verti-cal [0 0 0 1] direction. The subsequent lateralgrowth of GaN on the (1 11 0 1) facets resulted in thepropagation of the dislocations in the horizontaldirection, leading to the elimination of the thread-ing dislocations in the upper portion of the pyr-amid. The mechanism for forming the 903 bends isnot clear. In the high-defect-density region atthe center of the pyramid, many half-loops wereobserved as is usually seen in GaN epilayers (e.g.Refs. [15,16]).

By using smaller opening sizes, such as 1}2 lm,the defects could be con"ned to a much smaller

Fig. 5. Weak-beam dark-"eld image at a higher magni"cationtaken near the [1 1 21 0]GaN zone axis with u(5u) di!ractingconditions. u"0 0 0 2 revealing the dislocation distribution inthe core region of the GaN pyramid and near the interface withthe mask.

volume, resulting in a more e$cient reduction ofdefect density. Larger pyramids should also yielda larger amount of defect-free volume in the pyr-amid. The maximum size of the pyramids is deter-mined by the distance between the openings.Growing larger pyramids may be limited by thestability of the masking layer at the growth temper-ature and the chemical ambient over a longer peri-od of time. However, the possibility of growing thepyramids to 100 lm sizes is attractive because anentire active device of that size could be built ona polarized, defect-free pyramid.

4. Summary

In summary, GaN pyramids have been grown on(1 1 1)Si substrates by selective lateral overgrowthon a patterned Si

3N

4mask. A high-temperature

AlN bu!er layer grown at 11503C was found to becritical to initiate the mono-crystalline GaN/AlNseeding layer and subsequent pyramidal growth,consistent with a previous study on other substra-tes [12]. TEM studies revealed that the defect den-sity reduction was similar to that reported for

W. Yang et al. / Journal of Crystal Growth 204 (1999) 270}274 273

sapphire and SiC substrates. The threading disloca-tions originating at the GaN/AlN seeding layerwere found to bend through 903 and emerge at thesides of the pyramid giving a reduction and elim-ination of the upward propagation of the threadingdislocations. The results from this study suggestthat GaN selective lateral overgrowth can be per-formed on Si substrates and yield a crystal qualitywhich is similar to those grown on sapphire andSiC substrates.

Acknowledgements

This work was supported by the Air ForceWright Laboratory under Contract F33615-96-E-1973 monitored by Gary Smith. The authors wouldlike to thank Holly Marsh and Je! Ridley fortechnical assistance. Additional support at the Uni-versity of Minnesota was provided by NSF grantdDMR-9522253. The TEM is part of the CIECharacterization Facility at the University of Min-nesota.

References

[1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T.Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T.Kozaki, H. Umenoto, M. Sano, K. Chocho, Appl. Phys.Lett. 72 (1988) 2014.

[2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T.Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T.Kozaki, H. Umenoto, M. Sano, K. Chocho, Jpn. J. Appl.Phys. 36 (Part 2) (1997) L1568.

[3] O. Nam, M.D. Bremser, T.S. Zheleva, R.F. Davis, Appl.Phys. Lett. 71 (1997) 2683.

[4] T.S. Zheleva, O. Nam, M.D. Bremser, R.F. Davis, Appl.Phys. Lett. 71 (1997) 2472.

[5] B.H. Beaumont, S. Ha!ouz, P. Gibart, Appl. Phys. Lett. 72(1998) 921.

[6] R.D. Underwood, D. Kapolnek, B.P. Keller, S. Keller, S.P.DenBaars, U.K. Mishra, Solid-State Electron. 41 (1997)243.

[7] D. Kapolnek, S. Keller, R. Vetury, R.D. Underwood, B.P.Keller, S.P. DenBaars, U.K. Mishra, Appl. Phys. Lett. 71(1997) 1204.

[8] Y. Kato, S. Kitamura, K. Hiramatsu, N. Sawaki, J. CrystalGrowth 144 (1994) 133.

[9] H.M. Manasevit, F.M. Erdmann, W.I. Simpson, J. Elec-trochem. Soc. 118 (1971) 1864.

[10] M. Morita, S. Isogai, N. Shimizu, K. Tsubouchi, N.Mikoshiba, Jpn. J. Appl. Phys. 20 (1981) L173.

[11] T. Takeuchi, H. Amano, K. Hiramatsu, N. Sawaki, I.Akasaki, J. Crystal Growth 115 (1991) 634.

[12] A. Watanabe, T. Takeuchi, K. Hirosawa, H. Amano, K.Hiramatsu, I. Akasaki, J. Crystal Growth 128 (1993) 391.

[13] L. Wang, X. Liu, Y. Zan, J. Wang, D. Wang, D. Lu, Z.Wang, Appl. Phys. Lett. 72 (1998) 109.

[14] P. Kung, A. Saxler, X. Zhang, D. Walker, T.C. Wang,I. Ferguson, M. Razeghi, Appl. Phys. Lett. 66 (1995) 2958.

[15] M.T. Johnson, Z. Mao, C.B. Carter, Mater. Res. Soc.Symp. Proc. 482 (1998) 405.

[16] Z. Mao, M.T. Johnson, C.B. Carter, Microscopy Micro-anal. Proc. 4 Suppl. 2 (1998) 628.

274 W. Yang et al. / Journal of Crystal Growth 204 (1999) 270}274