High quality m-plane GaN grown under nitrogen-rich conditions by plasma assistedmolecular beam epitaxya)Marta Sawicka, Anna Feduniewicz-Żmuda, Henryk Turski, Marcin Siekacz, Szymon Grzanka, Marcin Kryśko,Igor Dzięcielewski, Izabella Grzegory, and Czesław Skierbiszewski Citation: Journal of Vacuum Science & Technology B 29, 03C135 (2011); doi: 10.1116/1.3589228 View online: http://dx.doi.org/10.1116/1.3589228 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/29/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy J. Appl. Phys. 115, 213506 (2014); 10.1063/1.4880957 Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy AIP Advances 4, 027114 (2014); 10.1063/1.4866445 Surface morphology evolution of m-plane ( 1 1 ¯ 00 ) GaN during molecular beam epitaxy growth: Impact ofGa/N ratio, miscut direction, and growth temperature J. Appl. Phys. 114, 023508 (2013); 10.1063/1.4813079 Mg doping of GaN grown by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions Appl. Phys. Lett. 96, 132103 (2010); 10.1063/1.3374882 High electron mobility GaN grown under N-rich conditions by plasma-assisted molecular beam epitaxy Appl. Phys. Lett. 91, 221905 (2007); 10.1063/1.2817597

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High quality m-plane GaN grown under nitrogen-rich conditions by plasmaassisted molecular beam epitaxya…

Marta Sawickab�

Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Polandand TopGaN Ltd., Sokolowska 29/37, 01-142 Warsaw, Poland

Anna Feduniewicz-Żmuda and Henryk TurskiInstitute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland

Marcin Siekacz and Szymon GrzankaInstitute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Polandand TopGaN Ltd., Sokolowska 29/37, 01-142 Warsaw, Poland

Marcin Kryśko, Igor DziJcielewski, and Izabella GrzegoryInstitute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland

Czesław SkierbiszewskiInstitute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Polandand TopGaN Ltd., Sokolowska 29/37, 01-142 Warsaw, Poland

�Received 26 October 2010; accepted 19 April 2011; published 18 May 2011�

Homoepitaxial growth of m-plane GaN �11̄00� as a function of substrate miscut and temperaturewas studied by plasma assisted molecular beam epitaxy �PAMBE�. The authors demonstrate that itis possible to obtain high-quality GaN on the m-plane under nitrogen-rich conditions at 730 °C.This is in contrast to the c-plane where three-dimensional growth mode is observed under the same

conditions. They find a strong growth anisotropy and describe GaN �11̄00� surface morphologydependence on the sample miscut direction. The results indicate that by introducing a sample miscut

toward �112̄6̄� one may expect parallel atomic steps when growing under nitrogen-rich conditionsat 730 °C by PAMBE. © 2011 American Vacuum Society. �DOI: 10.1116/1.3589228�

I. INTRODUCTION

Growth of GaN on the c-plane �0001� under nitrogen-rich�N-rich� conditions at temperatures below or about 750 °Cresults in rough surface morphology because three-dimensional �3D� growth mode is dominant,1,2 which resultsfrom very high diffusion barriers for Ga adatoms.3 In orderto improve the structural quality of the layers grown underN-rich conditions, much higher temperatures and nitrogenpressures are required to obtain morphologically flat surfaceswith atomic steps. It can be easily realized in ammonia–molecular beam epitaxy �MBE� or metal organic vapor phaseepitaxy but not in plasma assisted MBE �PAMBE� where notenough amount of active nitrogen is supplied by the plasmasource to stabilize the GaN surface at high temperatures. Asan alternative, it was demonstrated that for PAMBE, the useof metal surfactant allows to realize the step-flow epitaxy atmuch lower temperatures. Typically, high quality c-plane�0001� GaN can be grown by PAMBE under metal-richconditions.4,5 It was demonstrated that a thin metal layer onthe surface lowers the diffusion barriers6 and activatesatomic kinks5 on the c-plane which is quite inert.4 Therefore,high quality nitride structures can be grown by PAMBE atlow growth temperatures using an excess of Ga or In.

However, GaN exhibits a significant etching anisotropywhich indicates that other crystal orientations, such asN-polar c-plane or nonpolar, are less stable than the III-polar

c-plane, e.g., the etching rate of the m-plane �11̄00� in mol-ten KOH is significant, while for the c-plane it is negligible�except at defect sites where etch pits occur�.7 This fact pre-sents the concept to grow GaN on the m-plane surface underN-rich conditions at temperatures typical for growth of GaNon the c-plane under metal-rich conditions �about 750 °C�.

In this work we present the results of PAMBE homoepi-taxial growth under N-rich conditions on the m-plane andc-plane. We make a comprehensive analysis of the influenceof temperature and miscut on the m-plane GaN layer mor-phology, which is an important factor of its quality.

II. EXPERIMENT

The homoepitaxial growth of GaN layers was performedby PAMBE under N-rich conditions on bulk m-plane GaNsubstrates. Substrates were obtained from thick hydride va-por phase epitaxy �HVPE� boules that were cut in 10

�5 mm slices of m-plane �11̄00� orientation. Samples werepolished mechanically and chemomechanically. The averageroot mean square roughness �RMS� after polishing was 0.5nm over a 5�5 �m2 area as measured by atomic force mi-croscopy �AFM�. The substrate miscut value was nominally

0.5° toward �0001̄� and was confirmed by the x-ray diffrac-tion.

a�This article is based on material presented at the 27th North America Mo-lecular Beam Epitaxy Conference.

b�Electronic mail: sawicka@unipress.waw.pl

03C135-1 03C135-1J. Vac. Sci. Technol. B 29„3…, May/Jun 2011 1071-1023/2011/29„3…/03C135/5/$30.00 ©2011 American Vacuum Society

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In each experiment we grew 100–150 nm thick GaN lay-ers on both m-plane and c-plane substrates which weremounted together on one Mo back-coated 2 in. GaN /Al2O3

template by indium. Fluxes of gallium and nitrogen werekept constant for all the experiments, i.e., 3.7 and 6.2 nm/min, respectively. The resulting flux ratio �Ga/N� needed tobe 0.6 in order to be under N-rich conditions and only thetemperature was varied from 660 to 730 °C. Measurementof the substrate temperature was performed using a bandedge spectrometer BandiT from k-Space Associates.8 More-over, according to BandiT readings for the investigated tem-perature range in our V90 system, the sample surface tem-perature is proportional to the heater temperature measuredby the thermocouple on the backside of the wafer.

For exact determination of the crystallographic directionson the sample surface, we examined the samples cleavedalong �0001� direction. Each sample was placed on the AFMstage in a well-defined manner so that the AFM tip move-ment was either parallel to the �0001� direction or a 45°rotation in order to better visualize the atomic edges.

III. RESULTS AND DISCUSSION

A. GaN layer morphology on the m-plane and c-plane

The morphologies of m-plane and c-plane GaN layersgrown at 730 °C are shown in Fig. 1. On the m-plane weobserve an extremely smooth surface with atomic steps, incontrast to the c-plane GaN where 3D growth is observed.The roughness of the c-plane GaN layer is very high, with arms value of 13.9 nm measured over a 3�3 �m2 area. The

result of the growth on the c-plane agrees well with recentlyreported morphologies of c-plane GaN grown under N-richconditions by PAMBE.2

The AFM image of the m-plane GaN layer shows parallel

atomic steps flowing toward the �1̄1̄26̄� direction giving verylow surface roughness with a rms value of 0.1 nm only. It isimportant to stress that this is the first report of such a highquality, smooth GaN layer grown by PAMBE under N-richconditions. The atomic step edges are preferentially alignedperpendicular to the particular crystallographic direction

�112̄6̄�, or the mirror direction �1̄1̄26̄�. Armitage et al.9 car-ried out a series of m-plane GaN growths by PAMBE, andthey reported that the smoothest layers are obtained formetal-rich regime, while under N-rich conditions they ob-serve partially coalesced slatelike features and a 3D growthmode. The rms values, measured over a 2�2 �m2 area,were in a range from 1.3 to 4.3 nm. However, we cannotmake a direct comparison with our results because they used4H-SiC substrates and significantly lower growth tempera-ture.

B. Diffusion barrier anisotropy on the m-plane

Theoretical calculations of adatom kinetics on them-plane surface10 reveal a strong anisotropy of the barrierheight for Ga adatom diffusion which should affect the sur-face morphology. Figure 2�a� presents the calculated valuesfor the diffusion barriers of Ga adatoms on the m-plane, i.e.,

0.93 eV along �0001� and 0.21 eV along �112̄0�.10 The re-spective diffusion paths are also marked schematically.

The very low diffusion barrier along the a-direction andthe high diffusion barrier in the c-direction suggest a recom-mended miscut to create atomic step-edges along thea-direction. Following that theoretical recommendation, we

introduced a sample miscut of 0.5° toward �0001̄�. Figure2�b� presents the morphology of the layer grown at 730 °C.The surface is quite smooth, and we observe very dense zig-zag shaped atomic steps. Their edges are perpendicular to thetwo directions that are 17° away from the c-direction as mea-sured by AFM. Looking at the crystallographic structure of

the m-plane, we identified these directions to be �112̄6̄� and

�1̄1̄26̄�. In Fig. 2�b� the atomic steps that are perpendicular to

�1̄1̄26̄� appear to be longer than those perpendicular to

�112̄6̄� because the miscut direction is not exactly toward

�0001̄� but slightly tilted creating the disproportion of theedge lengths. The influence of the miscut direction on thesurface morphology will be discussed in Sec. III C.

Interestingly, if we compare Fig. 2�a� showing the theo-retically predicted diffusion paths for Ga adatoms on them-plane and Fig. 2�b� showing step edge shapes of them-plane GaN grown at 730 °C, it appears that the atomicedges tend to follow the direction of the diffusion paths pre-dicted theoretically despite the length scales being com-pletely different. However, this is only an impression be-

[000 ]1

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2

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]11

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[110]

2

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(b)

3 m�

3 m�

3 m�

3 m�

2 nm

80 nm

FIG. 1. �Color online� �a� Parallel atomic steps on m-plane GaN layer grownunder N-rich conditions at 730 °C. Local substrate vicinal angle �miscut� is

0.25° toward �112̄6̄�. The preferential direction of step-flow is marked

�1̄1̄26̄�. Steps are marked with dashed lines. �b� Morphology of c-plane GaNlayer grown in the same growth run shows evidence of 3D growth. Note theheight scales.

Sawicka et al.: High quality m-plane GaN grown under N-rich conditions by PAMBE03C135-2 03C135-2

J. Vac. Sci. Technol. B, Vol. 29, No. 3, May/Jun 2011

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cause the diffusion paths are perpendicular to �112̄4̄� whichis 22° away from �0001� direction, and, moreover, the barrier

for the Ga adatom to move straight along �112̄4̄� would betoo large for the Ga adatom to overcome.11

Nevertheless, it is a fact that the step edges on m-planeGaN grown under N-rich conditions at 730 °C are perpen-

dicular to �112̄6̄� directions instead of �0001�. For the layersgrown at lower temperatures, this is not the case, and we

observe the terraces perpendicular to �0001� �along �112̄0��.The difference in these two cases may be explained by thedependence of the step-flow velocity on the growth tempera-ture since the crystal shape is defined by the respective facetgrowth kinetics12,13 and the latter is dependent on the tem-perature. The growth conditions in PAMBE are, however,very far from equilibrium, and we did not carry out an analy-sis of the growth kinetics dependence on the temperature sowe can only speculate on the respective step velocity.

Looking at the temperature-dependence of the m-planesurface morphology presented in Figs. 2�b�–2�d�, we see thatapplying the same miscut on the layers grown at three dif-ferent temperatures yields smooth surfaces only for the lay-ers grown at 730 °C. At lower temperatures the diffusionbarriers play a more significant role, and it is impossible toget high-quality smooth layers under N-rich conditions �Figs.2�c� and 2�d��. The atomic steps can be seen on all of thesamples, although for lower temperature, they are not sosharp as compared to high temperatures. The atomic terraceedges are marked with white dashed lines in Figs. 2�b�–2�d�.

C. Influence of miscut on the m-plane morphology

We analyzed the influence of the miscut on the samplemorphology by choosing various substrate vicinal anglespointing toward the −c-, +c-, and a-directions. Introducing a

miscut toward �0001̄� produced atomic steps flowing in two

preferential step-flow directions: �112̄6̄� and �1̄1̄26̄�, as pre-sented in Fig. 3�a�. By orienting the miscut vector exactly

along �1̄1̄26̄�, which is a preferential step-flow direction, weobtained perfectly smooth surfaces with parallel atomic steps�Fig. 1�. We conclude also that a change in the miscut valuefrom 0.5° to 0.25° influences the terrace width but not thestep-edge shape.

The introduction of a miscut toward +c �0001� resulted ina so-called tilelike or slatelike morphology �Fig. 3�b�� thathas been already reported in literature.14 There are two kindsof atomic edges found: one along �0001� and a second,

shorter one, along �112̄0�. The second one appears when themiscut angle is not perfectly toward �0001�, but a smalla-component is present. Here we did not observe the steps

flowing toward �112̄6̄� or �1̄1̄26̄�, instead atomic edgestended to be straight and aligned along the above mentionedc- and a-directions.

We would like to note that the substrate misorientationtakes place before mechanical polishing, and any inaccuracyduring surface polishing may introduce an undesired compo-nent of the miscut vector toward the a-direction whichcauses surface roughening. We observed that it was verychallenging to achieve high accuracy in substrate surfacemisorientation. For the sample with a miscut toward −c, thisdid not play such a significant role, but it was important forthe +c-miscut sample with a strong tendency toward surfaceroughening.

FIG. 2. �Color online� �a� Schematic view of the m-plane surface with dif-fusion barriers for Ga adatoms calculated by ab initio �Ref. 10�. Diffusion

barrier along �1̄1̄26̄� is 0.21 eV and along �0001� is 0.93 eV. Diffusion pathsare marked with dashed lines. ��b�–�d�� Morphology of m-plane GaN layers

grown on substrates with miscut 0.5° toward �0001̄� under N-rich conditionsat different temperatures: �b� 730 °C—rms value 0.1 nm, �c� 700 °C—rmsvalue 1.8 nm, and �d� 660 °C—rms value 4.0 nm. The rms values corre-spond to 5�5 �m2 area scans.

Sawicka et al.: High quality m-plane GaN grown under N-rich conditions by PAMBE03C135-3 03C135-3

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When the miscut was toward the a-direction, in manycases, we observed small islands on the a-slopes �Fig. 3�c��.The shape of the islands reflects the growth anisotropy justdescribed: on the +c-slope we see zigzag shaped atomicedges and on the −c-slope there are rectangular-shape atomicedges. In-between we can see some atomic steps flowingtoward the a-direction.

D. High quality m-plane GaN

The high quality of m-plane GaN layers grown at 730 °Cunder N-rich conditions was confirmed by AFM measure-ments �Figs. 1�a� and 2�b��. The surface is extremely smoothwith an rms roughness of only 0.1 nm over a 5�5 �m2

area.Defect selective etching �DSE� was done on the layer

grown at 730 °C �Fig. 4�. The density of etch pits on thesurface of this sample was 2�107 cm−2, i.e., the same as forthe HVPE substrate. This confirms that no new defects weregenerated during the MBE growth of the 100 nm m-planeGaN layer.

The photoluminescence �PL� experiment at room tem-perature �Fig. 5� compares two m-plane GaN layers grown at730 and 660 °C. For the layer grown at 730 °C, the GaNband-to-band transition peak at about 364 nm is dominant,while for the layer grown at 660 °C, a so-called yellow lu-minescence at 580 nm is observed. The strong peak at 580nm is an evidence of the presence of defects that have beencreated during the growth at low temperature. The PL datagive an independent confirmation of the lower quality oflayers grown at the temperatures below 730 °C.

IV. CONCLUSIONS

We have studied the growth of GaN under N-rich condi-tions by PAMBE on the m-plane and c-plane and have foundthe conditions for growth of high quality m-plane GaN. Wepresented AFM measurements of the GaN layers that have avery low surface roughness and parallel atomic steps. De-creasing the growth temperature, we have observed the de-crease of the GaN layer quality, i.e., surface roughening, andevidence of the presence of defects. We discussed the depen-dence of morphology and step-flow mechanism on crystalmiscut direction. The study of the influence of miscut andtemperature on the surface morphology allowed us to iden-

tify the recommended miscut direction, i.e., �112̄6̄� or �1̄1̄26̄�for GaN growth at 730 °C. It is important to stress that

[11

]20

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0

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0

[11

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2

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1 �m

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(b)

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FIG. 3. �Color online� Morphology of the m-plane GaN layer for different

miscut directions: �a� toward �0001̄�, �b� toward �112̄0�, and �c� toward�0001�. Schemes on the right side show the corresponding miscut directionsfor the images on the left. The miscut vector, being a sample surface normal,

is slightly titled from �11̄00�. Its projection on the m-plane �11̄00� definesthe miscut direction in respect to �0001�.

2 �m

FIG. 4. �Color online� Etch pits on m-plane GaN layer grown at 730 °Cunder N-rich conditions by PAMBE. Small dots visible at the surface areresiduals of the surface cleaning after DSE.

350 400 450 500 550 600 6501E-4

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0,1

1

10PL @RT from m-plane GaN layer

grown at 730oC

grown at 660oC

Norm

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FIG. 5. �Color online� Photoluminescence spectra �RT� of m-plane GaNlayers grown at 730 and 660 °C.

Sawicka et al.: High quality m-plane GaN grown under N-rich conditions by PAMBE03C135-4 03C135-4

J. Vac. Sci. Technol. B, Vol. 29, No. 3, May/Jun 2011

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N-rich conditions are not used to grow polar nitrides byPAMBE, but they seem to provide good quality m-plane lay-ers. The presented results open new possibilities to this tech-nique by creating a new, wider window of the growth con-ditions, offering more parameters to play with, andeliminating the need to keep a stable metal layer during thegrowth.

ACKNOWLEDGMENTS

This work was supported partially by the Polish Ministryof Science and Higher Education Grant No. IT 13426 and theEuropean Union within European Regional DevelopmentFund, through grant Innovative Economy �Grant No.POIG.01.01.02-00-008/08� and within FP7-PEOPLE-IAPP-2008 through Grant No. SINOPLE 230765.

1C. Skierbiszewski, M. Siekacz, P. Perlin, A. Feduniewicz-Żmuda, G. Cy-wiński M. Leszczyński, I. Grzegory, Z. R. Wasilewski, and S. Porowski,J. Cryst. Growth 305, 346 �2007�.

2G. Koblmüller, F. Wu, T. Mates, J. S. Speck, S. Fernandez-Garrido, andE. Calleja, Appl. Phys. Lett. 91, 221905 �2007�.

3T. Zywietz, J. Neugebauer, and M. Scheffler, Appl. Phys. Lett. 73, 487�1998�.

4B. Heying, E. J. Tarsa, C. R. Elsass, P. Fini, S. P. DenBaars, and J. S.Speck, J. Appl. Phys. 85, 6470 �1999�.

5C. Skierbiszewski, Z. R. Wasilewski, I. Grzegory, and S. Porowski, J.Cryst. Growth 311, 1632 �2009�.

6J. Neugebauer, T. K. Zywietz, M. Scheffler, J. E. Northrup, H. Chen, andR. M. Feenstra, Phys. Rev. Lett. 90, 056101 �2003�.

7D. A. Stocker, E. F. Schubert, and J. M. Redwing, Appl. Phys. Lett. 73,2654 �1998�.

8J. J. Harris, R. Thomson, C. Taylor, D. Barlett, R. P. Campion, V. A.Grant, C. T. Foxon, and M. J. Kappers, J. Cryst. Growth 300, 194 �2007�.

9R. Armitage, M. Horita, J. Suda, and T. Kimoto, J. Appl. Phys. 101,033534 �2007�.

10L. Lymperakis and J. Neugebauer, Phys. Rev. B 79, 241308�R� �2009�.11L. Lymperakis, private communicaton �2010�.12G. Wulff, Z. Kristallogr. 34, 449 �1901�.13Q. Sun, C. D. Yerino, T. S. Ko, Y. S. Cho, I.-H. Lee, J. Han, and M. E.

Coltrin, J. Appl. Phys. 104, 093523 �2008�.14P. Waltereit, O. Brandt, M. Ramsteiner, A. Trampert, H. T. Grahn, J.

Menniger, M. Reiche, and K. H. Ploog, J. Cryst. Growth 227–228, 437�2001�.

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