TEM观察下研究生长在pss上的GaN的微观结构特点与位错演变

Microstructural properties and dislocation evolution on a GaN grown on patterned sapphire substrate:A transmission electron microscopy study

Y.H.Kim,1,a?H.Ruh,1Y.K.Noh,2,3M.D.Kim,3and J.E.Oh4

1Korea Research Institute of Standards and Science,1Doryong-Dong,Yuseong-Gu,Daejeon305-340,

Republic of Korea

2Wooree LST Corporation,Ansan-shi,Kyungki-do425-833,Republic of Korea

3Department of Physics,Chungnam National University,220Gung-Dong,Yuseong-Gu,Daejeon305-764,

Republic of Korea

4Division of Electrical and Computer Engineering,Hanyang University,Ansan City,Kyunggi-do425-791,

Republic of Korea

?Received5October2009;accepted21January2010;published online16March2010?

The microstructural properties of a GaN layer grown on a patterned sapphire substrate?PSS?were

studied in detail using transmission electron microscope techniques to determine dislocation and

growth behaviors.Regular and uniform recrystallized GaN islands were observed on the protruding

pattern.On a?at sapphire surface,the crystallographic orientation relationship of

?1ˉ21ˉ0?GaN on FS//?11ˉ00?sapphire and?11ˉ01?GaN on FS//?12ˉ13?sapphire existed between the GaN

and the substrate.On the other hand,the orientation relationship of?1ˉ21ˉ0?GaN layer//

?1ˉ21ˉ0?GaN island on IS//?11ˉ00?sapphire and?11ˉ01?GaN layer//?0002?GaN island on IS//?12ˉ13?sapphire was

con?rmed among the GaN layer,the recrystallized GaN islands on an inclined sapphire surface and

the PSS.The?at surface among the protruding patterns began to?ll rapidly with GaN.Then,the

GaN gradually overgrew the protruding pattern and coalesced near the summit as the growth time

increased.The generation of threading dislocations was observed in the vicinity of the coalescence

points near the top of the protruding patterns.?2010American Institute of Physics.

?doi:10.1063/1.3327004?

I.INTRODUCTION

Optical devices that use nitride-based compound semi-conductors,such as light-emitting diodes?LEDs?,laser di-odes,and photodetectors,have been developed to achieve high quality and ef?ciency.1–4In particular,many research groups have tried to reduce the density of threading disloca-tions and improve the quantum ef?ciency of these optical devices.Although nitride-based optical devices are commer-cially available,these are not high-power devices.Currently available nitride-based devices are usually manufactured on sapphire substrate.However,numerous threading disloca-tions are induced in the nitride-based layer.This is due to the large lattice mismatch,the large difference in thermal expan-sion coef?cients,and the chemical characteristics of GaN and sapphire,despite the use of low-temperature GaN and AlN buffers grown on a sapphire substrate.Although bril-liant blue-and-green LEDs have been created,despite their high dislocation density,the annihilation of the threading dis-location generated during the deposition process is desirable in order to improve the performance and reliability of these devices.Many new growth methods,such as the epitaxial lateral overgrowth?ELOG?,pendeoepitaxy,and facet-controlled ELOG,have been suggested as ways to improve the crystal quality by reducing the threading dislocation density.5–12However,these lateral overgrowth techniques are complex,for example,the deposition of SiO2and/or SiN.In

addition,impurity contamination and strain-induced defects

in the subsequent layer growth are highly likely.

Growing nitride compound semiconductors on a pat-

terned sapphire substrate?PSS?without a mask or interrup-

tion has recently been considered as an alternative way to

reduce the threading dislocations and enhance the light out-

put power as well as the internal quantum ef?ciency of light

re?ections through geometrical effects.2,13–16However,most

studies related to PSS have focused on evaluating the de-

vice’s capability and the reduction of dislocation density.Un-

til recently,the initial growth and the dislocation behaviors

for GaN grown on a dot-patterned sapphire substrate have

not been widely reported.In this article,we report on the

growth behavior and the microstructural properties and of

GaN layer grown on a PSS from a microstructural point of

view.

II.EXPERIMENT

GaN layers were grown on a plane and a patterned sap-

phire?0001?substrates using metalorganic chemical vapor

deposition?MOCVD?method.The sapphire substrates were

patterned by dry etching to form a protruding pattern array.

Dry etching was carried out by means of inductively coupled

plasma?ICP?of Cl2/Ar.After fabrication,we noted that the pattern was uniform in size and arranged well.The pattern’s

height and diameter were2and3.4?m,respectively.The distance between each pattern was4?m.This PSS was

a?Author to whom correspondence should be addressed.Electronic mail:

young.h.hkim@kriss.re.kr.FAX:?82-?0?42-868-5032.

JOURNAL OF APPLIED PHYSICS107,063501?2010?

0021-8979/2010/107?6?/063501/4/$30.00?2010American Institute of Physics

107,063501-1

etched with an H 2SO 4:H 2O 2?1:1?solution for 20min at 120°C to obtain a clean and oxide-free surface.After clean-ing,the PSS was heated to 1040°C under H 2ambient con-ditions for 10min and then etched thermally.To obtain an improved surface,we grew a 32-nm-thick GaN initiation layer on the PSS at 530°C.The GaN layer was then recrys-tallized at 1150°C for 5min.Subsequently,a GaN layer was grown at 1180°C under ambient pressure of 400mbar.During the growth process for the GaN layers,the ?ow rate of trimethylgallium ?TMGa ?and ammonia ?NH 3?were 160sccm and 10slm,respectively.To study the microstructural properties,we prepared TEM specimens using tripods for mechanical grinding and Ar-ion beam milling.Before load-ing the specimen,a plasma treatment was done to remove surface contamination and to avoid electron beam damage during the TEM operation.Bright-?eld TEM ?BFTEM ?im-ages,selected-area electron diffraction ?SAED ?patterns,and high-resolution TEM ?HRTEM ?micrographs were collected by using FEI F30microscopes operating at 300kV .Speci?-cally,the detailed interface structures between the GaN and the PSS were studied using TEM techniques.III.RESULT AND DISCUSSION

Figure 1shows SEM images of the GaN grown on plane and patterned sapphire substrates.Figures 1?a ?and 1?b ?are images taken of recrystallized GaN after it was deposited to a thickness of 32nm on plane and patterned substrates.Al-though a multiple-nucleation process was observed on both substrates,the degree of complexity was much more severe on the patterned substrate than the nucleation behavior was on the plane substrate.Recrystallized GaN islands were also observed on the protruding pattern,and these resembled twin elongated triangular pyramids with mirror symmetry.The GaN islands had a uniform shape across the surface of the substrate.The triangular structures were elongated along the ?112

ˉ0?direction,speci?cally as pairs with a threefold sym-metry.The ?at surfaces ?FSs ?among the protruding patterns ?lled rapidly with GaN,although nucleation and recrystalli-zation occurred across the surface area,as water ?lls the low-lying areas.Then,the GaN began to completely cover the protruding patterns as the growth time increased ?Fig.1?c ??.While ?lling in the protruding pattern,the GaN layer showed a sixfold symmetry that was consistent with the ?112

ˉn ?planes on the pattern.In Fig.2,three pairs of BFTEM images show cross-sectional views of a GaN layer grown on a PSS along the

?112

ˉ0?direction.These images were obtained under normal two-beam diffraction conditions to reveal the edge,screw,and mixed components of the threading dislocations.Figures

2?a ?,2?c ?,and 2?e ?correspond to g =?101

ˉ0?,where the speci-men has been tilted slightly from the ?112

ˉ0?zone axis along the plane perpendicular to the growth surface.Under these conditions,the basal planes can be seen in pro?le ?edge-wise ?.Figures 2?b ?,2?d ?,and 2?f ?were obtained for g =?0002?,and are parallel to the growth direction.The dislo-cation density was low in the GaN layer grown on FS of the PSS ?Figs.2?c ?and 2?d ??.On the other hand,the density was high near the summit of the protruding patterns.Threading dislocations are mainly observed above the protruding pat-terns of the PSS,and they propagate with the multiplication of threading dislocations,which are observed during the fol-lowed growth of the GaN layer ?Fig.2?e ??.Horizontal basal-plane dislocations are generated within the followed growth of the GaN layer on different basal planes.These disloca-tions make a change in their propagating direction and thread toward the surface.However,most of the dislocations are invisible in Fig.2?f ?,which was obtained for g =?0002?,in-dicating that their Burgers vector b is 1/3?112

ˉ0?.The stem dislocation,indicated by arrows in Fig.2?f ?,has mixed com-ponents.On the other hand,the GaN region grown on the FS of the PSS is nearly free from the threading dislocation ?Figs.2?c ?and 2?d ??.In this sense,the lateral overgrowth onto the protruding pattern is very ef?cient in reducing the threading defects in the GaN layer.In Fig.2,we observed the recrys-tallized GaN initiation layer as it grew into three-dimensional islands on the protruding patterns.In addition,from the contrast difference in the two-beam bright ?eld im-ages,we deduced that these initiation islands had a different crystallographic orientation relationship from the followed GaN layer.

Figure 3consists of cross-sectional BF and HRTEM mi-crographs and a SAED pattern taken to study the

microstruc-

FIG.1.SEM micrographs of the GaN surfaces grown by MOCVD:?a ?GaN initiation layer on the plane sapphire substrate,?b ?GaN initiation layer on the PSS,and ?c ?GaN layer deposited to a thickness of 1.6?m on the

PSS.

FIG.2.Two-beam BFTEM images showing the cross-section of a GaN

layer grown on the PSS:??a ?,?c ?,and ?e ??g =101

ˉ0and ??b ?,?d ?,and ?f ??g =0002.Multiplication and propagation of dislocations are observed at the peak of the protruding pattern in ?e ?.All the ?gures were taken from differ-ent locations in the same sample with the regular pattern in Fig.1?b ?.

tural properties of the recrystallized GaN islands and the GaN layer and the interface properties between the GaN and the PSS.In the magni?ed bright-?eld image in Fig.3?a ?,the recrystallized GaN initiation layer grew into an elongated island along the ridge of the protruding pattern.The interface between the ridge of the protruding pattern and the GaN is rougher than that between the FS of the substrate and the GaN layer.The top region of the GaN initiation island is ?at and the sides are rounded.The sapphire substrate causes a compressive strain in the GaN layer because the effective lattice constant of GaN ?in the case of GaN,a -axis lattice constant a eff,GaN =3.189??is larger than that of sapphire ?a eff,sapphire =2.747??.The SAED pattern in Fig.3?b ?was

taken along the ?112

ˉ0?zone axis of GaN,which has a wurtz-ite structure.Figure 3?b ?shows two sets of diffraction pat-terns.Since the lattice constants of GaN are larger than those of the sapphire substrate,the set of SAED patterns shown toward the inside of Fig.3?b ?belongs to the GaN layer,and the set of SAED patterns shown toward the outside belongs to the sapphire substrate.In addition,extra spots indicated by the vacant triangles were observed in the SAED pattern.These extra spots originated from the recrystallized GaN ini-tiation island,and the distance between two extra spots is

approximated to the spots on the GaN ?11

ˉ01?planes.How-ever,the interplanar spacing calculated from the extra spots is a little larger than 2.438?,the interplanar spacing of the ?11

ˉ01?planes.We concluded that the extra spots originated from the ?0002?planes of the recrystallized GaN island for which the interplanar spacing was approximated as 2.593?.The growth direction of the GaN initiation island was ?0002?

direction,although they were grown on the ridge of the is-land.In the SAED pattern,the ?0002?planes of the initiation

island are parallel to the ?11

ˉ01?planes of the GaN layer and the ?12

ˉ13?planes of the PSS.Figure 3?c ?shows the HRTEM micrograph taken at the interface between the recrystallized GaN island and the protruding pattern ?region I in Fig.3?a ??.In Fig.3?c ?,thick Moiréfringes with a thickness of 11.27?are observed at the interface between the recrystallized GaN

island and the protruding pattern.The ?12

ˉ13?planes of the protruding sapphire pattern are parallel to the ?0002?planes of the recrystallized GaN,as observed in the SAED pattern.

Moreover,the ?11

ˉ01?planes of the GaN layer are parallel to the ?0002?planes of the recrystallized GaN island,and the atomic arrangement of the GaN layer is very clear and free from any defects on the ?at boundary in Fig.3?d ?,with this image taken at the boundary between the recrystallized GaN island and the GaN layer ?region II in Fig.3?a ??.

Figure 4shows the HRTEM micrographs obtained at the interfaces between the GaN layer and the PSS ?interfaces I and II in Fig.3?a ??.Many basal plane stacking faults were observed in the GaN layer near the FS and these are type I stacking faults with the smallest energy due to low stacking disorder ?the translation vector R of the type I stacking fault

is 1/6?202

ˉ3??.A small variation of surface roughness on the FS and a disordered atomic arrangement at the interface were observed in the HRTEM micrograph in Fig.4?a ?.The irra-diation effect of a reactive ion beam on the sapphire substrate for a GaN layer was demonstrated by Kim et al.17,18The effects of the PSS can be considered from three aspects.Since the substrate was modi?ed by ICP of Cl 2/Ar during the dry etching process,the nucleation of GaN on the PSS was affected on a dry-etched surface.Second,the protruding pattern affected the diffusion of source materials and the ?ow of a precursor.In addition,the lateral growth of GaN contin-ued through the protruding pattern.Uniform distribution of the protruding patterns ensures microscopic and strain distri-bution in the GaN layer.Finally,variations of surface rough-ness throughout the PSS may contribute to the strain relax-ation of the GaN layer at a high-growth temperature.Mis?t strain caused by the lattice mismatch and the difference of the thermal expansion coef?cient between the GaN and the sapphire substrate may be relieved by geometrical effects and chemical interactions between the ion-modi?ed substrate and its precursors.In the HRTEM study,an intermixed

re-

FIG.3.?a ?Magni?ed BFTEM image of recrystallized GaN island between the GaN layer and the PSS.?b ?SAED pattern showing the orientation rela-tionship between the inclined recrystallized GaN island and other structures ?the GaN layer and the PSS ?.?c ?HRTEM micrograph of the region I be-tween the inclined GaN island and the IS of the PSS in ?a ?.?d ?HRTEM micrograph of the region II between the inclined GaN island and the GaN layer in ?a ?

.

FIG.4.HRTEM micrographs showing the interface properties between the GaN layer and the PSS:?a ?Interface I between the GaN and the FS of the PSS in Figs.3?a ?and 3?b ?Interface II between the GaN and the IS of the PSS in Fig.3?a ?.

gion with a thickness of about 10nm was observed between the GaN layer and the ridge area of the protruding pattern,although these were not identi?ed in the bright-?eld images.Speci?cally,in Fig.4?b ?,a tilted GaN grain and many Moire fringes were observed between the GaN layer and the ridge area.The grain was tilted approximately 17°from the GaN layer.

IV.CONCLUSION

From these results,we concluded that a GaN grew through three steps.The recrystallized GaN islands were generated on the PSS.Then,the GaN began to ?ll a FS between protruding patterns.Finally,the GaN layer gradu-ally overgrew the protruding pattern and coalesced near the summit.The crystallographic orientation relationship of ?1ˉ21ˉ0?GaN layer //?1ˉ21ˉ0?GaN island on IS //?11ˉ00?sapphire and ?11ˉ01?GaN layer //?0002?GaN island on IS //?12ˉ13?sapphire

was con?rmed among the GaN layer,the recrystallized GaN is-lands on the inclined sapphire surface ?IS ?,and the PSS.The generation of threading dislocations was observed in the vi-cinity of the coalescence points,which were near the top of the protruding patterns.Vertical repropagation of disloca-tions was observed during the followed growth on the coa-lesced site.When the GaN met near the top region of the protruding pattern,the strain by the lattice mismatch between

GaN and sapphire substrate and the difference of thermal expansion coef?cient was concentrated at the coalesced site.At advanced stages of relaxation,multiplication of disloca-tions may dominate the strain relaxation process ?Fig.5?.Details of these mechanisms are currently under investiga-tion and will be reported later.ACKNOWLEDGMENTS

This work is supported by Korea Research Council of Fundamental Science and Technology ?KRCF ?through the project of “Development of Characterization Techniques for Nano-materials Safety.”

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FIG.5.?Color online ?Schematic diagram illustrating the microstructural properties and the dislocation behavior in the GaN grown on the PSS:?a ?low dislocation density in the GaN layer on a ?at sapphire surface,?b ?orientation relationship among the GaN layer,the inclined recrystallized GaN islands on the IS of the PSS,and the PSS;?1ˉ21ˉ0?GaN layer //?1ˉ21ˉ0?GaN island on IS //?11ˉ00?sapphire and ?11ˉ01?GaN layer //

?0002?GaN island on IS //?12ˉ13?sapphire ,and ?c ?dislocation behaviors observed

near the peak of the protruding pattern.

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