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GaN rods grown on Si by SAG-HVPE towards GaN HVPE/InGaN MOVPE core/shell structures Geoffrey Avit, Yamina ANDRE, Catherine Bougerol, Dominique Castelluci, Amélie Dussaigne, Pierre Ferret, Stephanie Gaugiran, Bruno Gayral, Evelyne GIL, Yann Lee, M. Réda Ramdani, Elissa Roche, and Agnes Trassoudaine Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01244 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016

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Crystal Growth & Design

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GaN rods grown on Si by SAG-HVPE towards GaN HVPE/InGaN MOVPE core/shell structures

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Geoffrey Avit‡, □, Yamina André ‡, □,*, Catherine Bougerol▲,◊,Dominique Castelluci‡,□, Amélie

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Dussaigne▲,°, Pierre Ferret▲,°, Stéphanie Gaugiran,▲,°, Bruno Gayral▲,●,Evelyne Gil ‡, □, Yann

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Lee▲,°, M. Reda Ramdani‡, □, Elissa Roche‡, □ and Agnès Trassoudaine‡, □,■

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‡

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Université Clermont Auvergne, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000 Clermont-Ferrand, France. □CNRS, UMR6602, Institut Pascal, F-63178 Aubière, France. ▲ Univ. Grenoble Alpes, F-38000 Grenoble, France. ◊CNRS, Institut Néel, F-38042 Grenoble, France. °CEA LETI, Minatec campus, 17 rue des martyrs, F-38054 Grenoble cedex 9, France. ● CNRS, Institut Néel, F-38042 Grenoble, France. CEA, INAC-SP2M, CEA-CNRS group Nanophysique et Semiconducteurs, F-38000 Grenoble, France ■ Institut Universitaire de Technologie, Dept. Mesures Physiques, Université d’Auvergne, 63172

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Aubière cedex, France

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KEYWORDS.GaN, SAG-HVPE, core/shell, micro-PL

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ABSTRACT

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Selective area growth hydride vapor phase epitaxy (SAG-HVPE) and metal organic vapor phase epitaxy (MOVPE) processes are coupled for the synthesis of high quality c-axis InGaN/GaN core/shell structures. The core consists in high aspect ratio GaN rods grown by SAG-HVPE on patterned N-polar AlN on Si(100) substrates. The shell is grown by MOVPE which provides abrupt InGaN/GaN multi-quantum wells (MQWs). Microphotoluminescence (µ-PL) analysis performed on the HVPE GaN core exhibit a narrow emission line of 3 meV in linewidth associated to the neutral-donor bound exciton revealing the excellent optical properties of the GaN material. For the core/shell wire geometry, the silane free HVPE process ensured the whole lateral cladding of the core. The hybrid HVPE core/MOVPE shell structures exhibit high optical quality without yellow luminescence.

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Corresponding Author

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Dr. Associate Prof. Yamina ANDRE Institut Pascal UMR 6602 CNRS / UBP / IFMA Campus des Cézeaux, 24 Avenue des Landais BP 8002663171 AUBIERE Cedex FRANCE Phone : +33473407587Fax : +33473407340 mail : [email protected]

(a)

4 µm

Main figure: (a) Post-growth SEM images of InGaN/GaN MQWs grown on a GaN rod. (b) HAADF-STEM image showing the ACS Paragon Plus Environment growth of InGaN/GaN MQWs. The InGaN part appears as bright.

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GaN rods grown on Si by SAG-HVPE towards

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GaN HVPE/InGaNMOVPE core/shell structures

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Geoffrey Avit‡, □, Yamina André ‡, □,*, Catherine Bougerol▲,◊,Dominique Castelluci‡,□, Amélie

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Dussaigne▲,°, Pierre Ferret▲,°, Stéphanie Gaugiran,▲,°, Bruno Gayral▲,●,Evelyne Gil ‡, □, Yann

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Lee▲,°, M. Reda Ramdani‡, □, Elissa Roche‡, □ and Agnès Trassoudaine‡, □

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‡

Université Clermont Auvergne, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000 Clermont-Ferrand, France. □CNRS, UMR6602, Institut Pascal, F-63178 Aubière, France. ▲ Univ. Grenoble Alpes, F-38000 Grenoble, France. ◊CNRS, Institut Néel, F-38042 Grenoble, France. °CEA LETI, Minatec campus, 17 rue des martyrs, F-38054 Grenoble cedex 9, France. ● CNRS, Institut Néel, F-38042 Grenoble, France. CEA, INAC-SP2M, CEA-CNRS group Nanophysique et Semiconducteurs, F-38000 Grenoble, France

KEYWORDS.GaN, SAG-HVPE, core/shell, micro-PL

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ABSTRACT

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Selective area growth hydride vapor phase epitaxy (SAG-HVPE) and metal organic vapor phase

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epitaxy (MOVPE) processes are coupled for the synthesis of high quality c-axis InGaN/GaN

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core/shell structures. The core consists in high aspect ratio GaN rods grown by SAG-HVPE on

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patterned N-polar AlN on Si(100) substrates. The shell is grown by MOVPE which provides

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abrupt InGaN/GaN multi-quantum wells (MQWs). Microphotoluminescence (µ-PL) analysis

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performed on the HVPE GaN core exhibit a narrow emission line of 3 meV in linewidth

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associated to the neutral-donor bound exciton revealing the excellent optical properties of the

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GaN material. For the core/shell wire geometry, the silane free HVPE process ensured the whole

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lateral cladding of the core. The hybrid HVPE core/MOVPE shell structures exhibit high optical

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quality without yellow luminescence.

ACS Paragon Plus Environment

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INTRODUCTION

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Recently, 1D c-axis core/shell rod structures with InGaN/GaN multi-quantum wells (MQWs)

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growing on the lateral m-planes of the inner rod have emerged as good candidates for LED

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applications.(1)(2) Heteroepitaxy on highly mismatched substrates is possible as strain relaxation

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is enhanced by the wire morphology. Moreover crack and dislocation densities can be drastically

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reduced, inducing less non-radiative recombination and therefore improving light emission

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efficiency.(3) Particularly, high aspect ratio columns offer a larger active zone per substrate unit

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area than planar layers in the case of core/shell structures.(4) The radial structure leads to a

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decrease of the quantum-confined Stark effect by reducing the piezoelectric polarization along

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the polar c-axis growth direction. The fabrication of such longitudinal InGaN/GaN MQWs in

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nanowires (NWs) has been demonstrated by self-assembled metalorganic vapor phase epitaxy

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(MOVPE) without catalyst to obtain NW-based LED devices, which is the standard industrial

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technique for nitride device fabrication. (4-6) But for this self-assembled process, issues appear for

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device engineering as the rods are not fully uniform in their diameter or density distributions,

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which leads to a dispersion in the optoelectronic properties.

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In order to obtain GaN rods with high aspect ratio, uniform dimensions and controlled

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positions, which is mandatory for large scale device integration, selective area growth (SAG) has

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been proposed as an efficient process. In SAG, the growth selectively occurs on patterned

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substrates in the precisely defined apertures of an inert dielectric mask, often SiNx(7)(8),

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SiOx(9)(10)or TiOx(11)(12). SAG of GaN rods on patterned silicon remains limited. Calleja et al.(9)

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have shown that GaN rods grown by plasma-assisted molecular beam epitaxy (PA-MBE) on

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silicon through a SiO2 mask were not single crystalline and that parasitical nucleation of GaN on

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the dielectric mask was difficult to control. In order to improve the crystalline quality of the GaN


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rods, a thin AlN or GaN buffer layer is often used between the silicon substrate and GaN

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rods(8)(10)(11)(13). In MOVPE, addition of silane in the gas phase is a common technique to obtain

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high aspect ratio GaN rods at high growth rate (up to 145 µm.h-1).(14) In this case, it has been

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shown that a thin SiN layer grows alongside the (10-10) m-planes(5), passivating the

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aforementioned vertical sidewalls. A major consequence is a thicker SiN layer at the bottom of

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the rods than at the top(5), because the bottom part is exposed longer to Si. So overgrowth by

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ternary alloy (In, Ga)-N required for the realization of a LED structure is found to be limited at

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the bottom of the rods(4-6), hindering the crucial advantage of core-shell geometry which is a very

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large active surface per substrate unit area.

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While the selective growth of GaN rods has been performed by MOVPE or MBE on various

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substrates (15), selective growth of GaN rods by hydride vapor phase epitaxy (HVPE) on AlN/Si

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substrates remains hitherto unreported despite potential remarkable features in terms of III-V

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alloy engineering and structure shaping. In this paper, we propose to combine MOVPE and

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HVPE processes for the synthesis of hybrid c-axis InGaN/GaN core/shell structures: InGaN/GaN

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MQWs shell is obtained by MOVPE on GaN rods grown by HVPE. Growth of InGaN/GaN

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MQWs shell with abrupt interfaces is well-mastered by MOVPE while a GaN core with both

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high aspect ratio and high optical quality is provided by the SAG-HVPE process. The attractive

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asset of HVPE for the growth of GaN based core/shell structure is then demonstrated with the

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overgrowth of core/shell InGaN/GaN MQWs by MOVPE along the entire length of GaN rods

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grown by HVPE. The optical properties of the HVPE GaN rods and the core/shell structures are

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investigated through microphotoluminescence (µPL) measurements.

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HVPE is a near equilibrium growth process, with chloride molecules as element III precursor,

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ideally suited to SAG of III-V semiconductors. For usual growth temperature (700 °C-1000 °C),


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there is no adsorption of chloride species on the dielectric mask and therefore no competition

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between masked and opened areas. Therefore the filling factor can be set high or low and does

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not play any role during growth. The growth is governed by surface kinetics, so that the mask

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pattern and the vapor phase composition can be freely tuned and consequently various shapes

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can be obtained as shown by our group(16)(17), even at the nanometer scale(18)(19), mainly

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depending on the intrinsic growth anisotropy of the crystal facets in the considered growth

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conditions. The growth can either be slow (3 µm.h-1) or very fast (up to 100 µm.h-1).

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EXPERIMENTAL SECTION

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GaN columns were grown in a 2 inches horizontal home-designed HVPE quartz reactor at (20)

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atmospheric pressure described in

.The input partial pressures of ammonia, gallium chloride,

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hydrogen chloride and hydrogen , , and were fixed to 1.05.10-1 atm, 6.67.10-

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3

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the growth time was 30 minutes. The growth of GaN rods was performed on silicon (100)

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substrates covered by a 100 nm thick N-polar AlN oriented along the c axis but textured in the

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plane(21). Polarity was measured by convergent beam electron diffraction (CBED) and by

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piezoelectric polarization measurements. Then, a 50 nm thick SiO2 mask was deposited by

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plasma-enhanced chemical vapor deposition (PECVD). The intermediate AlN layer is used for

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accommodating the lattice misfit and the thermal coefficient difference between the silicon

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substrate and the GaN and also for preventing any reaction between Ga species and the silicon

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substrate. The pattern consists of arrays of circular apertures with 0.7 µm diameter holes and 7

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µm in periodicity. The filling factor is then 0.01.

atm, 1.41.10 atm and 1.94.10 atm respectively. The growth temperature was 980 °C and

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MOVPE TEGa and TMIn precursors were used to grow the InGaN alloy of the MQWs shell

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with growth conditions comparable to 2D layer(4). The expected In content is 15%. The growth

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was not intentionally selective because high V/III ratio was used in order to incorporate In. It is

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known that high V/III ratio favors 2D growth, i.e. lateral growth rate. Finally, even with these

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specific growth conditions, InGaN deposition occurs only on wire top and wire sidewalls.

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RESULTS AND DISCUSSION

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Figure 1. SEM images of GaN rods grown on Si(100)/AlN /SiO2 substrate: (a) on a zone

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with 0.7 µm diameter holes. (b) A single GaN rod. (c) The same GaN rod after KOH

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etching.

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SEM images of the post growth patterned substrate are provided in Figures 1.a and 1.b. For the

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given experimental growth conditions, GaN rods perpendicular to the substrate are synthesized

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with a GaN rod emerging from each aperture. The rods grow along the (0001) direction and

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exhibit a hexagonal shape, delimited by the six low growth rate (1-100) m-plane with a (0001)

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plane as their top facet. The rod width (w) is the inner diameter measured from facet to facet.

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Few rods exhibit a none-flat top surface. Angles measurements performed with SEM show that

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these facets mainly consist of semipolar (1-101) r-planes. GaN rods exhibit two different

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alignments with a difference of 30 °. This behavior has been well documented

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issue for rod structures as long as there is no coalescence. The rod average growth rates are 30

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(±4) µm.h-1 and 4 (±1) µm.h-1 for the {0001} and {10-10} facets respectively. The aspect ratio

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of a GaN rod is defined as the ratio between the rod height (h) and the rod width (w). One should

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notice that width consists on the sum of the lateral expansion of the GaN rod and the diameter of

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the original aperture. With an aperture of 0.7 µm, h=15 µm and w=4.7 µm. Aspect ratio is

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calculated equal to 3.

(22)

and is not an

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The polarity of the GaN rods was investigated by KOH (35 %) etching at 40 °C for 2 min.

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KOH selectively reacts towards Ga-polar and N-polar GaN, Ga-polar GaN remaining untouched

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and N-polar GaN being etched. Twenty rods were randomly chosen on the substrate and

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observed with SEM before and after wet KOH etching. As shown as example in Figure 1.c and

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1.d, KOH etching reveals that each rod has simultaneously a Ga-polar domain and a N-polar

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domain. As reported by X.Wang et al.(23), N and Ga polarities can appear during the selective

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growth of GaN rods by MOVPE on SiOx/sapphire substrate, with Ga-polar material apparently

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formed during the nucleation on the SiOx mask. To circumvent this issue and to avoid the

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formation of Ga-polar c-facet, the authors adopted a truncated pyramid followed by a column


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growth mode, leading to single N-polar GaN rods. For the HVPE GaN core, the Ga-polarity does

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not seem to be only restricted to the nucleation on SiO2 mask but, as it is shown in Figure 1.c, it

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can also be observed inside the perimeter of the aperture. We suggest that the mixed polarity of

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GaN rods originates from the slightly textured AlN layer, generating nuclei of Ga and N polarity.

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Another approach to obtain single polar GaN rods would be to use a Ga-polar substrate to grow

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the GaN rods. In fact, it appears that, at least in MOVPE, GaN rods grown on a SiO2/ GaN-Ga

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polar/sapphire substrate can be single Ga-polar(24).

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In MOVPE one-step silane free processes have been developed to grow high aspect ratio GaN

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rods. An evidence of the influence of the growth temperature and the total reactor flow on the

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rods morphology has been shown. High quality GaN can be obtained but the growth rate is rather

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low (4 – 6 µm.h-1)(25)(26). Another approach consists in a two-step growth process with first a

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nucleation step at low temperature and then an annealing and a regrowth of N polar GaN at

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higher temperature. But the growth rate is still low (4 µm.h-1)(27).

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In our experiment and in HVPE growth of 3D structures in general, the shape is governed by

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vapor-solid deposition rates depending on the temperature and the vapor phase composition. In

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the case of this study, tuning the GaN rods aspect ratio is achieved by adjusting several

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experimental parameters, among them the additional HCl and H2 partial pressures which play a

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critical role to maintain the growth anisotropy between (0001) top facet and (10-10) vertical m-

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planes greater than 1. For example, without additional HCl, a 30 minutes growth time is

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sufficient for GaN rods to coalesce. No silane flux is required to promote axial growth over

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radial growth which induces a high quality of the grown GaN with a more homogeneous material

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from bottom to top with no inhomogeneous SiN passivation layer as reported in MOVPE(5).This


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should lead to a better light intensity per substrate unit area for core-shell structures. That is the

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great advantage of HVPE for the large scale device processing of vertical core-shell structures.

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Figure 2. (a) Microphotoluminescence spectrum at 4 K of GaN rods grown on

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Si(100)/AlN/SiO2. (b) High resolution spectrum near the NBE.

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Optical properties of the GaN rods were investigated through µPL spectroscopy. µPL

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measurements on GaN rods grown in the conditions of Figure 1 were performed in a helium flow

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cryostat at T=4 K, by using an excitation wavelength of 244 nm. The laser beam was focused

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onto the sample on a 1 µm diameter spot and the laser power was 1 µW on the sample.

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Reproducibility of the measurements was assessed by performing µPL on two samples grown in

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the same conditions and on many rods for each sample. Similar results were found and no


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difference was observed between each zone on a same sample. A typical µPL spectrum recorded

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between 3.20 eV and 3.60 eV is given in Figure 2.a. Three transitions are observed: a near band

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edge (NBE) emission at 3.47 eV, the donor acceptor pair (DAP) around 3.27 eV and an emission

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line around 3.46 eV. In Figure 2.b, a high resolution µPL spectrum between 3.40 eV and 3.52 eV

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is displayed. D°X(A), D°X(B) and FX(A) transitions can be isolated at respectively 3.472 eV,

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3.476 eV and 3.479 eV. This latter value is compatible with a strain free GaN material, according

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to the 3.478 eV measured by Torri et al.(28) for strain free GaN. The small full width at half

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maximum (FWHM) of about 3meV of the D°X transition shows a wurtzite GaN material with

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good crystalline quality and low strain inhomogeneity, while the low intensity DAP peak around

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3.27 eV indicates a rather low residual doping. The µPL spectrum of GaN rods grown on GaN

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buffered Si(111) patterned substrates by MBE by Albert et al.(11) has a FWHM for the D°X

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transition of 6.5 meV at 12 K. Choi et al.(29) reported a linewidth of 81 meV for the near band-

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edge emission of MOVPE-grown GaN wires on masked (0001) sapphire substrates with a 0.5

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µm thick GaN buffer layer at 3.7 K. A weak emission at a lower energy around 3.46 eV is found

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(Figure 2.b), possibly due to doping or inversion domain boundaries (IDB) separating adjacent

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domains of Ga- and N-polarity as reported in bulk structures(30-32) and more recently in PA-MBE

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grown GaN nanowires(33)(34). This is consistent with the mixed N and Ga polarity of the GaN rod,

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revealed by KOH etching in Figure 1.c.


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Figure 3. (a) Post-growth SEM images of InGaN/GaN MQWs grown on a GaN rod. (b)

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HAADF-STEM image showing the growth of InGaN/GaN MQWs. The InGaN part

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appears as bright. (c) High-resolution HAADF-STEM image of one of the InGaN QW

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taken along the [11-20] zone axis.

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The attractive feature of GaN rods obtained by SAG-HVPE for LEDs will now be

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demonstrated. C-axis InGaN/GaN MQWs core/shell structures have been synthesized by

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MOVPE on GaN rods grown by HVPE. During the MOVPE growth, the targeted shell thickness

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and In content were 200 nm and 15% respectively(4). The growth of the entire core/shell structure

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by HVPE is not possible. The III-chloride precursors are synthesized inside the reactor far from

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the substrate. So the control of gas flows during transient stages does not allow the growth of

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abrupt interfaces as MBE or MOVPE are used to.


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The post-growth SEM image displayed in Figure 3.a shows a full coverage from InGaN/GaN

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MQWs along the m-planes of the GaN rods, by the way maximizing the total active area,

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contrary to what can often be seen for a same structure grown entirely by the MOVPE process(4-

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6)

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Focus Ion Beam (FIB) showing the five InGaN/GaN QWs grown on the GaN rod. At higher

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magnification (Figure 3.c) we can see that the InGaN quantum wells are well-defined, 4.3 nm

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wide in average; along the growth direction they present a sharp bottom interface with GaN with

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atomic steps between terraces whereas the top interface with GaN is more diffuse. Regarding the

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optical properties and as point of comparison, InGaN/GaN MQWs core/shells structures have

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also been grown under the same conditions on GaN rods by MOVPE as in (4).The height and the

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diameter of these rods were respectively 15 µm and 1.6 µm. PL spectra of InGaN/GaN structures

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grown by MOVPE on HVPE (red line) or MOVPE (black line) GaN rods are given in Figure 4.

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PL is collected at 300 K using an excitation wavelength of 325 nm. The impinging laser power is

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9.5 mW for a spot of 200 µm in diameter. A broad peak related to InGaN MQWs emission

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centered at 2.775 eV (448 nm) and 2.733 eV (454 nm) can be observed on both samples. The

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high optical quality from the HVPE (core)/MOVPE (shell) sample is attested by the GaN NBE

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emission around 366 nm and the absence of yellow luminescence, contrary to the sample fully

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grown by MOVPE. The fact that we do see GaN band-edge luminescence in the HVPE core /

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MOVPE shell sample indicates that the photo-created carriers which have not diffused towards

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the InGaN QWs have a significant probability to recombine at the GaN band-edge. In the

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MOVPE only sample, these same carriers who have not diffused towards the InGaN QWs

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recombine either non-radiatively or at the yellow-band emission.

. Figure 3.b displays a HAADF-STEM image of a rod prepared in longitudinal cross section by

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Figure 4. Photoluminescence spectrum of InGaN/GaN structures grown by MOVPE on

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GaN rods grown by HVPE (red line) or MOVPE (black line). PL intensities of the two

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spectra should not be compared due to the density difference between the grown structures

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(1.2 107 cm-² for the core/shell fully grown by MOVPE and 1.2 106 cm-² for the hybrid

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HVPE core / MOVPE shell).

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CONCLUSION

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In conclusion, hybrid high optical quality c-axis InGaN/GaN core/shell structures have been

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synthesized thanks to MOVPE and HVPE processes. SAG-HVPE of GaN rods on patterned

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polycrystalline AlN-N polar/Si(100) substrates was performed. Fifteen micrometers long (0001)

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oriented GaN rods were obtained. Thanks to the near equilibrium HVPE process, one can take

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advantage of the intrinsic crystal growth anisotropy through direct tuning of the vapor phase

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composition. No addition of silane is required to promote axial growth versus radial growth. µPL

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measurements show state-of-the-art optical qualities with a 3 meV FWHM for the D°X transition

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and no yellow luminescence often observed for MOVPE grown GaN rods. Full cladding by

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InGaN/GaN MQWs obtained by MOVPE is achieved on non polar m-planes leading to a large


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active surface. These structures do not exhibit yellow luminescence and are promising candidates

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as building blocks for LED applications.

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AUTHOR INFORMATION

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Corresponding Author

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[email protected]

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

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to the final version of the manuscript.

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ACKNOWLEDGMENT

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We acknowledge support from FIDEL ANR French Project (ANR-NANO-029 01) and from

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GANEX (ANR-11-LABX-0014). GANEX belongs to the public funded

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‘Investissements d’ Avenir’ program managed by the French ANR agency. This work was also

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supported by Région Auvergne and European FEDER grants (CPER 2013).

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REFERENCES

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FOR TABLE OF CONTENT USE ONLY

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GaN rods grown on Si by SAG-HVPE towards GaN HVPE/InGaN MOVPE core/shell structures

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Geoffrey Avit, Yamina André, Catherine Bougerol, Dominique Castelluci, Amélie Dussaigne,

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Pierre Ferret, Stéphanie Gaugiran, Bruno Gayral, Evelyne Gil, Yann Lee, M. Reda Ramdani,

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Elissa Roche and Agnès Trassoudaine

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SYNOPSIS In this work we have developed an original hybrid growth procedure for the growth of c-axis InGaN/GaN MQWs core/shell structures. It is based on SAG-hydride vapor phase epitaxy (SAGHVPE) and MOVPE. Full cladding by InGaN/GaN MQWs obtained by MOVPE is achieved on non polar m-planes of a high crystalline and optical HVPE GaN core leading to a high active surface.


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InGaN

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