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Initial stage of cubic GaN for heterophase epitaxial growth induced onnanoscale v-grooved Si(001) in metal-organic vapor-phase epitaxyTo cite this article: S C Lee et al 2019 Nanotechnology 30 025711

 

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Initial stage of cubic GaN for heterophaseepitaxial growth induced on nanoscalev-grooved Si(001) in metal-organic vapor-phase epitaxy

S C Lee1,5 , Y-B Jiang2, M Durniak3, C Wetzel3,4 and S R J Brueck1

1Department of Electrical and Computer Engineering and Center for High Technology Materials,University of New Mexico, Albuquerque, NM 87106, United States of America2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131,United States of America3Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180,United States of America4Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY12180, United States of America

E-mail: sclee@chtm.unm.edu

Received 27 August 2018, revised 9 October 2018Accepted for publication 19 October 2018Published 9 November 2018

AbstractThe initial stages of the nucleation of cubic (c-) GaN in heterophase epitaxy on a Si v-groove areinvestigated. Growth of GaN on a nanoscale {111}-faceted v-groove fabricated into a Si(001)substrate proceeds in the hexagonal (h-) phase that induces a secondary v-groove replicating thesubstrate topography with two opposing {0001} facets. The secondary v-groove is thenorientationally mismatched at the junction of the h-GaN facets (h–h junction) resulting instructural instability. This instability is relieved either by the formation of voids that reduce theactual junction area or by the transition to c-phase (h–c transition) suppressing further extensionof the h–h junction. The distribution of voids that is locally affected by the island growth modeof h-GaN on Si(111) and the imperfection in the groove geometry impacts the initial stage ofheterophase epitaxy. Primarily, The h–c transition is observed as a non-local phenomenon; itoccurs homogeneously and simultaneously along the bottom of the entire secondary groove andforms a one-dimensional (1D) seed layer except for some interruptions where the h–h junction isdefected by gaps or incomplete voids. Between these interruptions, epitaxy retains a singlecrystal but results in a series of c-GaN nanodots on the seed layer with large fluctuation in sizeand spacing. The adatom incorporation observed in this heterophase epitaxy is a 1D analog to thewetting of a substrate followed by the self-assembly in conventional quantum dot epitaxy. Thesurface morphology of the c-GaN nanodots is governed by the faceting mostly composed of(001)- and (11n)-orientations and the roughening between these facets that ultimately affect themorphology of the final top surface of the c-III-N. The interruptions interfere with thehomogeneity of the h–c transition and can cause antiphase defects and mosaicity. Based onexperimental results, a solution to improve these issues is proposed.

Keywords: cubic GaN on Si(001), phase transition, v-groove, Stranski–Krastanov mode, metal-organic vapor-phase epitaxy

(Some figures may appear in colour only in the online journal)

Nanotechnology

Nanotechnology 30 (2019) 025711 (6pp) https://doi.org/10.1088/1361-6528/aae9a2

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For light emitting diodes (LEDs), the (100) surface in thecubic (c-) phase of III-N compound semiconductors is anonpolar facet that does not exhibit the piezoelectric effectsand impacts the efficiency of emission by removing thedegradation associated with them [1]. However, the c-phaseIII-N is difficult to grow since it is energetically unfavorablecompared with the h-phase [2]. This problematic issue resultsin random phase mixing in a single epilayer for conventionallarge-area epitaxy. There have been several attempts to growc-III-Ns relying on planar substrates having identical crys-tallinity and small lattice mismatch such as SiC, or having thesame crystallinity but large lattice mismatch such as GaAsand Si, requiring a thick buffer or a thin SiC interlayer.Additionally, patterned epitaxy for c-III-N on these substrateshas been examined [3–5]. While some of these attemptsreported noticeable progress, most of them have not yielded apractical solution for device fabrication.

III-N on Si has a strong potential for integration with Simicroelectronics. Radically different from previous attempts,we have demonstrated epitaxial growth of large, isolated areasof c-GaN on a v-grooved Si(001) substrate [6, 7]. The growthproceeds with sequential growth of h- and c-GaN with orien-tation- and phase-dependent incorporation respectively andachieves c-InxGa1-xN/GaN quantum wells (QWs) mono-lithically on a Si(001) substrate with h-GaN as an interlayerbetween them [8–10]. The layer structure for this procedure isschematically illustrated in figure 1. In this figure, a critical stepis the phase transition from h- to c-phase (h-c transition) thatrelieves structural mismatches accumulated in the h-GaN on a

Si v-groove particularly at the junction of two misorientedh-GaNs (h–h junction indicated with an arrow). The h–ctransition abruptly occurs atop the h–h junction along thegroove, leads to the spatial phase separation of metastablec-GaN from stable h-GaN, and as a result allows their coex-istence with spatial phase separation in a single layer. Thedetails of the growth mechanism have been reported elsewhere[11]. By growing c-InxGa1-xN/GaN QWs oriented with its(001) facet aligned to that of the Si substrate and fabricating anLED, we have demonstrated green emission at the wavelengthof 500 nm, free from piezoelectric effects [8].

Although the basic growth mechanism for c-III-N hasbeen reported [11], the h–c transition and the associated initialstage of c-GaN growth has yet to be fully elucidated. Tocomplete the phase transition (or nucleation of c-GaN), thelocal atomic arrangement must go through several steps thatimpact the crystallinity of the c-phase with sequential corre-lation. In this work, we focus on the very initial stage of c-GaN growth that is crucial to its crystallinity and surfacemorphology. Scanning and transmission electron micro-scopies (SEM and TEM) and scanning tunneling microscopy(STM) were used for three-dimensional structure analysis.The faceting and surface morphology associated with theinitial stage of the phase transition is clearly revealed, and anexperimental approach to improving the results for deviceapplications is proposed.

An array of v-grooves used for GaN growth was fabri-cated into a Si(001) substrate by i-line interferometric litho-graphy and KOH-based anisotropic etching with Cr films as

Figure 1. A schematic cutaway illustration of the heterophase epitaxy of GaN on a v-grooved Si(001) substrate. While epitaxy begins with h-phase (blue) in each v-groove, it abruptly transits to c-phase (green) with the nucleation of the c-GaN at the bottom of the secondary v-grooveformed by h-GaN which replicates the (111)-faceted Si v-groove with (0001) facets. The discontinuity of the c-GaN nucleation by thepresence of an (open) void at the h–h junction is depicted. The details of the faceting on the surface and the formation of nanodots in the c-GaN are omitted for simplicity.

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an etch mask. The period of the array and the width of asingle v-groove were 4 μm and 900 nm, respectively. Metal-organic vapor-phase epitaxy (MOVPE) was employed forGaN growth. A thin AlN buffer layer was used at thebeginning of growth to assist the nucleation of GaN onSi(111) facets inside each groove. The growth was stoppedimmediately after the h–c transition. The details of the patternfabrication and growth conditions used in this work have beensummarized elsewhere [11].

Figure 2 provides (a) a top-down view SEM image of asingle groove and cross sectional (b) TEM and (c) STMimages along the center of the groove indicated by two whitearrows at both ends in (a). It reveals the initial stage of c-GaNclearly; the nucleation of c-GaN (or abrupt phase transitionfrom h- to c-GaN during epitaxy) occurs atop the h–h junctionhomogeneously along the bottom of the secondary v-grooveand forms nanoscale dots (nanodots). The spatial continuityof this nucleation is locally interrupted by some spots wherethe top of the h–h junction is defected by an interruption (agap or a hole that disrupts the continuity of the initialv-groove) between facing h-GaN layers. The insets offigure 2(b) in the red outline and 2(c) in the blue outlineindividually match the regions between the color codeddashed lines in 2(a) and show enlarged views. In each inset, avoid is formed at the h–h junction which is not closed beforethe h–c transition and, therefore, remains as a gap at thebottom of the secondary groove. Clearly, any h–c transitionoccurring along the groove bottom is interrupted by this gap

and the crystal continuity is also interrupted. In the two-dimensional (2D) view at the cross section along the h–hjunction, most of the voids including the interruption in theinset of figure 2(b) retain a random shape while some of themsuch as that in the inset of figure 2(c) are partly enclosed bywell-defined facets. Our previous article focused on theinhomogeneous nucleation of c-GaN at the early stage of theh–c transition with the formation of nanodots by irregularagglomeration of Ga adatoms [11]. This work presents moreprecise observation based on the non-local characteristics ofthe h–c transition homogeneously providing a c-phase seedlayer atop the h–h junction along the groove bottom. It shouldbe noted that the early stage of c-GaN formation in figure 2 isvery similar to a one-dimensional (1D) analog of the 2Dwetting layer prior to the self-assembly of quantum dots(QDs), referred to as the Stranski–Krastanov (S–K) mode,while the dot scale is considerably different [12].

Between these interruptions, the h–c transition proceedssimultaneously and homogeneously, and the c-GaN generatedat the groove bottom is a single crystal with a coherent crystalorientation. This is crucial in understanding the mechanism ofphase transition and the resulting crystallinity. Because ofthese gaps that inevitably exist at the groove bottom, the c-GaN does not remain as a single crystal along the entiregroove. Instead, it could suffer from antiphase defects andmosaicity associated with multiple initiation regions thatcoalesce in continued growth. However, such gaps are rela-tively rare. In an examination along the groove bottom with

Figure 2. (a) A top-down SEM image that reveals the initial stage of the c-GaN nucleation on a secondary v-groove by h-GaN. Note theinhomogeneous filling of the bottom of the groove. (b) A TEM and (c) an STM cross sectional image along the bottom of the secondaryv-groove that follows the line connecting the white arrows at both ends of (a). Inset of (b) (left panel outlined with red in the middle): amagnification of the red box pointed by an arrow in (b) which matches the area between the red dashed lines in (a). Inset of (c) (right paneloutlined with blue in the middle): a magnification of the blue box pointed by an arrow in (c) which matches the area between the blue dashedlines in (a). Each inset shows the discontinuity of the c-GaN nucleation disturbed by an unclosed void at the top of the h–h junction. Thegreen dashed line in each inset follows the h–c boundary. The regions A and B at the bottom under (c) represent regions of low and high voiddensity, respectively. It should be noted that (b) and (c) are the composite of multiple images. (d) and (e) SAED patterns taken from theregions of the c-GaN indicated by the arrows in (b) which are located between interruptions shown in the insets of (b) and (c). The whitearrow in each SAED corresponds to (002) spot.

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the distance up to ∼100 μm by TEM, only six spots wereclearly observed from the present sample. Relying on thisobservation, the average interruption spacing is at least∼10 μm and the c-GaN in figure 2 roughly maintains singlecrystalline characteristics up to this length scale. The selectivearea electron diffraction (SAED) patterns in figures 2(d) and(e) which were obtained from the regions between interrup-tions support the single crystallinity of the c-GaN6.

Epitaxy continued on this quasi-1D seed layer is char-acterized by an irregular agglomeration of Ga adatoms thatresults in a series of nanodots along the groove bottom with alarge size fluctuation ranging from less than 50 nm up to∼500 nm. Thus at the initial stages of growth, the c-GaNconsists of a homogeneously thin layer along the groovebottom with randomly spaced multi-disperse nanodots whichform along the seed layer. The amount of c-GaN in the thinnucleation is primarily correlated with the distribution of thevoids generated at the h–h junction. Figures 2(b) and (c)reveal the relation between void distribution and the c-GaNgrowth; there is less c-GaN growth where the voids are moredensely distributed (i.e. region B) and vice versa (i.e. regionA). As reported in previous work, void formation means localrelief of the structural instability at the h–h junction andtherefore the reduction of the resulting stress. In contrast, thearea of the h–h junction with negligible voids is relativelyunstable. The spatial variation of the instability associatedwith void distribution then drives Ga adatoms to migrate fromlow-to high-stressed areas (e.g. away from regions with a highdensity of voids) atop the h–h junction along the groovewhere multi-dimensional strain relaxation processes such asdot formation are available [13, 14]. The adatom incorpora-tion observed in the h–c transition is therefore analogous tothe S–K mode mentioned earlier, typically observed in largelattice mismatch heteroepitaxy. On the other hand, the scaleof the dot partly affected by the lattice mismatch of c- to h-GaN is ∼0.002 along the [111]-[0001] at the given growthcondition, not large enough to induce a few nm-scale con-ventional QDs [9].

The irregular agglomeration of c-GaN at the initial stagecontributes to the roughening of the top surface after addi-tional growth that can combine with surface undulation.While the top surface of the c-GaN ultimately evolves to(100) orientation as the groove fills, a certain degree offluctuation in flatness during epitaxy is unavoidable. Infigure 2(a), the nanodots extend with stripe patterns across thegroove in the top-down view. The cross sectional TEM andSTM in figures 2(b) and (c) reveal that the surface is boundedby faceting with various orientations and the rougheningbetween facets in each dot. In figure 3 which is a TEMmagnification of the white boxes in figures 2(b) and (c), someof the facets are identified as {11n} (n= 1, 3, K) and (001)but the most frequently observed is {111}. While it is not

clear whether this orientation is Ga- or N-terminated, itsdominance among the facets observed on the surface impliesit has the lowest surface free energy at the present growthconditions. This is consistent with the reported data of othercubic III-V semiconductors [15]. All the facets on each dotare unexceptionally perpendicular to the groove direction andtheir boundaries appear as a stripe pattern in top-down viewof figure 2(a). Also, every nanodot in this figure is elongatedalong the groove. These imply uneven stress on a c-GaN dotthat is correlated to the 1D groove shape. Beside faceting, asindicated by white bold arrows in figure 3 for example, sur-face roughening without clear facet orientation is observed.This could be due to the collision between different neighborfacets during growth.

In figure 3, the c-GaN is defected by stacking faults (SFs)which are randomly distributed within each nanodot. Thisfigure reveals that SF generation is not directly correlated withfaceting or roughening. Mostly it is related with strain reliefand imperfections on the h-GaN since the SFs can bedescribed as a fluctuation between c- and h-phases. As arough estimate, the average spacing is below 100 nm, quitedifferent from the ∼10 μm between gaps discussed above.This implies that SF generation is a partial relief process ofthe strain within a single domain of c-GaN7. Also, the surfaceundulation of which the period is roughly sub-μm scaleobserved in previous work could be another strain reliefavailable within a single c-phase domain [11].

Atomic level flatness of the c-GaN surface at the top iscritical to high-performance devices for Si-based c-III-Nelectronics and photonics. However, the top surface of the c-GaN is degraded by sub-μm scale undulation and nm-scalefaceting, and is far away from the ideal flatness. There areseveral steps such as groove fabrication, h-GaN growth, andh–c transition which impact the flatness of the top surface.The anisotropic etching for Si v-groove must be strictly

Figure 3. A magnification of the white box in figure 2(b) thatmatches the area between the white dashed lines in (a) with theidentification of facet orientation and stacking faults. White boldarrows point the areas of surface roughening between facets. Thegreen dashed line means the h–c boundary.

6 Because of slight bending by the stress on the long (>10 μm), thin(<50 nm) specimen and probably a large number of stacking faults, the sizeand intensity of the diffraction spots in figures 2(d) and (e) are not quiteuniform. However, they provide an evidence for single crystallinity of the c-GaN between interruptions which are confirmed by the insets of figures 2(b)and (c).

7 As reported in our previous articles, the c-GaN grown at the inside of thesecondary v-groove is in tensile stress with negligible lattice mismatch of∼0.002 at the phase interface.

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controlled to maintain high homogeneity in groove geometry.Two single Si(111) facets with minimal kinks or steps mustbe provided as a starting point of h-GaN growth. Well-established process technology of Si microelectronics cansatisfy this requirement with precisely oriented, defect-freeSi(001) substrates. On the other hand, the nucleation of h-GaN on Si is not highly controlled because of the largemismatch in crystallinity as well as lattice constant. Its islandgrowth mode on Si(111) induces fluctuation of the structuralinstability at the h–h junction, which directly affects thehomogeneity of the h–c transition along the groove bottom[16]. This problem that originates from the intrinsic char-acteristics of constituting materials is very difficult to resolve,the formation of gaps along the groove bottom therefore doesnot arise solely from as-fabricated sample imperfections.

A possible experimental direction to resolve the rough-ening of the top surface and the crystal degradation is thecontrol of the h–c transition with nucleation proceeding from asingle dot. Here, single dot means c-GaN nucleated with uni-form h–c transition on a nanoscale limited area. The basic ideais to reduce of the length of the v-groove until it becomescomparable with its width. Then, the resulting shape is similarto an inverted pyramid with slight extension in one direction asshown in figure 4(a). This simply controls the h–h junctionlength available for the h–c transition and can allow single dotformation on a limited area at the bottom. Figure 4(b) is a

top-down SEM image of a single dot of c-GaN nucleated in aregion bounded by four h-GaN nanocrystals which was grownunder the conditions similar to those applied to figure 2. Infigure 4(a), the width and length of the secondary groovemeasured at the top are ∼550 nm and ∼620 nm, respectively.The apparent length of the h-GaN secondary v-groove at thebottom is ∼50 nm, evidently much smaller than the ∼10 μm ofthe average interruption spacing but comparable to the nanodotsize and the SF spacing. The growth of GaN on this Si grooveproduces h-GaN that symmetrically covers two pairs of facingSi(111) facets. While it incurs another type of h–h junctionindicated by a white arrow at every corner in figure 4(b) whereneighboring h-GaNs meet each other, epitaxy provides a well-defined, four-side secondary groove with h-GaN{0001} at thetop without further geometric complexity.

By keeping the length slightly greater than the width inthe given v-groove, the h–c transition available in an infinitelylong v-groove can be simply reproduced over a small region.Unlike the dots in the long v-groove in figure 2, the single dotin figure 4(b) does not exhibit a stripe pattern across it clearly,implying the absence or reduction of multiple faceting on thesurface and therefore supporting the heterophase epitaxythrough the h–c transition with the surface morphology ofc-GaN improved at the groove bottom. In figure 4(a), the areaavailable for c-GaN(001) at the top is slightly less than 1×1μm2 avoiding surface undulation. As illustrated in figure 4(c),the c-GaN after complete fill-up is ready for planar processingwith nearby Si devices and large enough for integrated cir-cuits as well as discrete devices with the current 10 nm-classCMOS node technology. As suggested in previous work, theextension of the secondary v-groove width comparable to theadatom migration length at a given growth condition isavailable and therefore μm-scale c-GaN is also achievable[11]. Further study on the c-GaN from a single dot nucleationfor practical applications is presently under way.

In summary, the initial stage of c-GaN nucleation onh-GaN grown in a Si v-groove by MOVPE has been investi-gated. The h- to c-phase transition to relieve the structuralinstability at the h–h junction occurs abruptly and the c-GaNnucleates homogeneously along the bottom of the entire sec-ondary groove. The homogeneity of the c-phase nucleation isdisturbed by some interruptions such as gaps or discontinuitiesof the h–h junction with an average spacing of ∼10μm in thegiven sample. Between each of these interruptions, the c-GaNretains single crystalline characteristics but results in a series ofrandom-size nanodots along the groove bottom at the initialstage. The dot formation is driven by the migration of Gaadatoms across the spatial variation of the stress associated withvoid distribution on the h–h junction which locally relieves thestructural instability. The c-GaN 1D nucleation homogeneouslycovering the groove bottom followed by the formation ofnanodots is quite analogous to the wetting and self-assembly ofQDs known as S–K mode. In contrast, c-GaN nanodots areindividually degraded by nm-scale faceting and roughening insurface morphology and SFs in crystallinity. For practicalapplications, a single dot nucleation of c-GaN in a groove of thelength limited by the width has been demonstrated to addressthese issues.

Figure 4. (a) A top-down SEM image of a secondary v-groove by h-GaN with a length is slightly greater (∼50 nm) than the width. Thetop surface of all four sides of the h-GaN is {0001}. (b) A top-downSEM image of a c-GaN nanodot nucleated at the bottom of thesecondary v-groove similar to that in (a). White arrows point to thejunctions of the h-GaNs individually grown on neighboring Si(111)facets that meet each other orthogonally in a plane along the Si(001)facet. (c) A schematic projection of the epitaxy of a single c-GaNnanodot on the left panel that completely fills up the inside of thesecondary v-groove in continued growth on the right panel withbetter surface morphology and crystallinity.

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Acknowledgments

This work was supported primarily by the EngineeringResearch Centers Program (ERC) of the National ScienceFoundation under NSF Agreement No. EEC-0812056 and inpart by New York State under NYSTAR Contract No.C090145.

ORCID iDs

S C Lee https://orcid.org/0000-0003-3283-6016C Wetzel https://orcid.org/0000-0002-6055-0990S R J Brueck https://orcid.org/0000-0001-8754-5633

References

[1] Xie J, Ni X, Fan Q, Shimada R, Ozgur U and Morkoc H 2008Reduction of efficiency droop in InGaN light emittingdiodes by coupled quantum wells Appl. Phys. Lett. 93121107

[2] Yeh C-Y, Lu Z W, Froyen S and Zunger A 1992 Zinc-blende–wurtzite polytypism in semiconductors Phys. Rev. B 4610086–97

[3] Kemper R M, As D J and Lindner J K N 2013 Silicon-basedNanomaterials ed H Li (New York: Springer) ch 15

[4] Wang D, Hiroyama Y, Tamura M and Ichikawa M 2000Growth of hexagonal GaN on Si(111) coated with a thin flatSiC buffer layer Appl. Phys. Lett. 76 1683–5

[5] For review, see Morkoc H 2009 Handbook of NitrideSemiconductors and Devices (New York: Wiley) andreferences therein

[6] Lee S C, Sun X Y, Hersee S D, Brueck S R J and Xu H 2004Spatial phase separation of GaN selectively grown on ananoscale faceted Si surface Appl. Phys. Lett. 84 2079–81

[7] Lee S C, Pattada B, Hersee S D, Jiang Y-B and Brueck S R J2005 Nanoscale spatial phase modulation of GaN on av-grooved Si substrate—Cubic phase GaN on Si(001) forMonolithic Integration IEEE J. Quantum Electron. 41596–605

[8] Stark C J M, Detchprohm T, Lee S C, Jiang Y-B,Brueck S R J and Wetzel C 2013 Green cubic GaInN/GaNlight-emitting diode on microstructured silicon(100) Appl.Phys. Lett. 103 232107

[9] Lee S C, Youngblood N, Jiang Y-B, Peterson E J, Stark C J M,Detchprohm T, Wetzel C and Brueck S R J 2015Incorporation of indium on cubic GaN epitaxially inducedon a nanofaceted Si(001) substrate by phase transition Appl.Phys. Lett. 107 231905

[10] Durniak M T, Bross A S, Elsaesser D, Chaudhuri A,Smith M L, Allerman A A, Lee S C, Brueck S R J andWetzel C 2016 Green emitting cubic GaInN/GaN quantumwell stripes on micropatterned Si(001) and their strainanalysis Adv. Electron. Mater. 1 1500327

[11] Lee S C, Jiang Y-B, Durniak M T, Detchprohm T,Wetzel C and Brueck S R J 2016 Atomic-scale phasetransition of epitaxial GaN on nanostructured Si(001):activation and beyond Cryst. Growth Des. 16 2183–9

[12] Stranski I N and Krastanov L 1938 Zur theorie der orientiertenausscheidung von ionenkristallen aufeinander Akad. Wiss.Wien. 146 797–810

[13] Konkar A, Madhukar A and Chen P 1998 Stress-engineeredspatially selective self-assembly of strained InAs quantumdots on nonplanar patterned GaAs(001) substrates Appl.Phys. Lett. 72 220–2

[14] Alonso-Álvarez D, Alén B, Ripalda J M, Rivera A,Taboada A G, Llorens J M, González Y, González L andBriones F 2013 Strain driven migration of In during thegrowth of InAs/GaAs quantum posts APL Mater. 1 022112

[15] Moll N, Kley A, Pehlke E and Scheffler M 1996 GaAsequilibrium crystal shape from first principles Phys. Rev. B54 8844–55

[16] Daudin B, Widmann F, Feuillet G, Samson Y, Arlery M andRouviere J L 1997 Stranski-Krastanov growth mode duringthe molecular beam epitaxy of highly strained GaN Phys.Rev. B 56 R7069–72

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