Growth and characterization of GaN NWs on various

sapphire substrates

Carina B. Maliakkala, A. Azizur Rahmana, Nirupam Hatuia, BhagyashreeA. Chalkea, Rudheer D. Bapata, Arnab Bhattacharyaa

aDepartment of Condensed Matter Physics and Materials Science, Tata Institute ofFundamental Research, Homi Bhabha Road, Mumbai 400005, India.

Abstract

GaN NWs were grown on c-plane, m-plane and r-plane sapphire substratesin a showerhead metalorganic chemical vapor deposition system using nickelcatalyst with trimethylgallium and ammonia as precursors. We studied theinfluence of carrier gas, growth temperature, reactor pressure, reactant flowrates and substrate orientation in order to obtain thin GaN NWs. Thenanowires grew along the <1011> and <1010> axes depending on the sub-strate orientation. These nanowires were further characterized using photo-luminescence and Raman spectroscopy.

Keywords: GaN, nanowire, MOCVD, nickel catalyst

1. Introduction

There has been a lot of interest in Gallium nitride (GaN) nanowires(NWs) nanoscale photonic [1, 2, 3, 4, 5, 6, 7, 8] and electronic [9, 10, 11,12, 13] device applications. The growth of GaN NWs by catalyst mediated[9, 10, 14, 15, 16] as well as catalyst free [7, 17, 18, 19] methods have beenreported. Unlike InAs, where <111> oriented wires are usually obtained[20, 21, 22, 23], III-nitride NW growth has been observed along differentcrystal directions and is strongly dependent on growth conditions.

The direction of NW growth can depend on the substrate, its orienta-tion, as well as the growth conditions. Ji et al. [24] reported the growth ofwires along the a-axis (<1120>) on c-plane sapphire with nickel as catalyst.Gottschalch et al. reported the growth of vertical wires on c-plane sapphiregrowing along c-axis <0001> using gold as catalyst [14]. On r-plane sapphireusing nickel-catalysed MOCVD, Zhou et al. obtained wires tilted on the sub-strate and growing along m-axis (<1100>) [15], but Wang et al. reportedthe growth of vertical wires along the a-axis (<1120>) [16]. Qian et al. re-ported the growth NWs along the a-axis (<1120>) on r-plane sapphire with

Preprint submitted to Elsevier July 7, 2015

nickel as catalyst [5]. Apart from the recent report of inclined GaN nanorodsgrown on m-plane sapphire along the c-axis (<0001>) without any catalyst[19], there are no reports on the growth of GaN NWs on m-plane sapphiresubstrates. In addition, there is no report comparing the growth of nickel-catalysed GaN NWs on c-, m- and r-plane sapphire substrates at identicalconditions. We studied the dependence of growth direction on the substrateorientation with other growth conditions kept the same, and characterizedthe samples using SEM, TEM, PL and Raman Spectroscopy.

2. Growth of nanowires

GaN NWs were grown using metalorganic chemical vapor deposition (MOCVD)with nickel catalyst and trimethylgallium (TMGa) and ammonia (NH3) asprecursors. The substrates were cleaned, drop-coated with nickel nitratehexahydrate solution (∼ 0.01 M), dried and annealed in hydrogen to formmetallic nickel nanoparticles which subsequently served as the catalyst parti-cle [16, 25]. This method of using nickel nitrate hexahydrate solution to getthe catalyst particles is faster than techniques involving evaporation of nickelor gold, and is a cheaper alternative to using commercially available goldnanoparticle suspensions. The size of the catalyst particle can be controlledby the annealing time and temperature (See Supplementary Information sec-tion I for details). Since the exact composition and phase (solid/liquid) of thenickel-gallium alloy that serves as the catalyst during growth is unknown, theNW growth mechanism could be either vapour-liquid-solid (VLS) or vapour-solid-solid (VSS) [15]. The growth of NWs was sensitive to the reactor con-ditions. We varied the temperature, reactor pressure and rate of flow ofprecursors in order to obtain thin and non-tapering NWs. The growth tem-perature was varied between 820 ◦C and 1020 ◦C. Experiments were alsocarried out at different reactor pressures between 50 torr and 200 torr. Verylow flow of precursors (∼ 2.5 sccm of TMGa and ∼ 1 sccm of NH3) andsmall V/III ratio (∼ 4.5) was used to facilitate anisotropic growth. We alsostudied the effect of carrier gas on growth. After annealing the nickel ni-trate hexahydrate coated sapphire substrates in a hydrogen environment toproduce nickel nanoparticles, we grew GaN NWs in a pure nitrogen ambient(figure 1(a)). When we tried to grow under similar reactor conditions in anH2 ambient (figure 1(b)) or in a mixture of equal volumes of N2 and H2,we did not obtain any wires. Instead, we obtained just droplets containingnickel and gallium, unlike earlier reports [11]. NWs were grown on differentorientations of sapphire namely c-plane (0001), r-plane (1102) and m-plane(1010) as shown in figure 1(c-e). On all these substrates we obtained NWsas thin as 20 nm diameter with a triangular cross section. We found that

2

1 µm 1 µm

(a) (b)

(c) (d) (e)

2 µm 2 µm2 µm

Figure 1: Growth of GaN NWs: (a) GaN NWs obtained by growing in N2 environment(840 ◦C and 150 torr with 1 sccm NH3 flow and 2.5 sccm TMGa flow). (b) Dropletscontaining gallium and nickel obtained by growing under H2 ambient at the similar growthconditions. (c-e) Nickel catalysed non-tapering GaN NWs obtained by growing under sameconditions (840 ◦C and 150 torr with 1 sccm NH3 flow and 2 sccm TMGa flow) on (c)c-plane (d) m-plane and (e) r-plane sapphire substrates.

a pressure of 150 torr in N2 environment and ∼ 840 ◦C was optimum forobtaining thin non-tapering wires. We also tried to grow GaN NWs usinggold nanoparticles as catalyst. Gold catalysed wires grew slower than theirnickel assisted counterparts under similar growth conditions, as reported byZhou et al. [15].

3. Structural characterization of nanowires

The crystal structure and the crystallinity of the NWs were analyzed usingX-ray diffraction (XRD) and Transmission electron microscopy (TEM).

3.1. XRD analysis

The grazing incidence XRD pattern the GaN nanowires grown on c-planesapphire substrates is shown in Figure 2. The peaks are quite sharp indicat-ing good crystal quality. The peak positions have been indexed to wurtziteGaN structure. By least square fitting the peak positions in the pattern

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were indexed to a hexagonal wurtzite structure. The lattice constants ob-tained by least square fitting from the peak positions were a = 3.188 A andc = 5.176 Awhich agrees well with values reported in literature.

3 0 4 0 5 0 6 0 7 0(20

-21)(11

-22)

(10-13

)

(11-20

)

(10-12

)

(10-11

)(00

02)

(10-10

)

Co

unts

(arb.

units)

2 θ ( d e g r e e )Figure 2: X-ray diffraction pattern with the planes indexed showing that the GaN wireshave crystallized in the wurtzite phase.

3.2. TEM analysis

(c)

5 nm

-1

(b) (a)

5 nm 5 nm

(1010)

(1010)

_

_

Figure 3: TEM of NWs: (a) TEM image showing a catalyst particle at the tip of theNW indicating that the growth mechanism is VLS or VSS. (b) High resolution TEM imageof a NW grown on c-plane sapphire. (c) Electron diffraction pattern of the same wire asin (b)viewed along the [0001] zone axis, where the line indicates axis of wire growth. Wehence infer that the wires grown on c-plane sapphire grow along <1010> i.e. m-axis.

The crystallinity and growth direction of the wires grown on c-, m- andr-plane sapphire were determined using transmission electron microscopy

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(TEM). Since the NW thickness is less than 100 nm they are electron trans-parent and do not require further thinning down for TEM studies. Thesample on which wires were grown was ultrasonicated in methanol to make asuspension. This suspension was slowly dropped multiple times onto a TEMgrid and the solvent allowed to evaporate. The catalyst nanoparticle seen atthe end of the wire in the TEM image (figure 3 (a)) indicates that the growthwas nickel catalysed. The lattice planes seen in the high resolution TEM im-ages (figure 3(b)) and the well-defined electron diffraction pattern obtained(figure 3(c)) confirm the single crystal nature of these wires. The diffractionpattern was indexed to find the lattice planes that gave rise to the diffractionspots. By comparing the diffraction pattern and the low magnification TEMimage of the wire, the growth axis was determined. The wires grew alongthe m-axis (<1010>) on both c-plane (figure 3(c)) and r-plane sapphire sub-strates. On m-plane sapphire most of the NWs grew along the <1011> and<1010> directions. An indexed electron diffraction pattern obtained with aTEM is shown in Figure 4 showing that the growth direction is <1011> forthis wire. The reports the growth of GaN NW on sapphire along <1011> israre, if not nonexistent. But growth along this semi-polar direction has beenreported on graphite, where the growth direction was seen to be temperaturedependent.[26]

5 nm-1

(1011)

(1011)

(1102)

(0111) -

- -

--

Figure 4: Electron diffraction pattern of a wire grown on m-plane sapphire when viewedalong the [1101] zone axis. The line indicates axis of wire growth we see that this wire hasgrown along the <1011>.

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Substrate Growth PL FWHMorientation direction (meV)

c-plane <1010> 220m-plane <1011>, <1010> 350r-plane <1010> 390

Table 1: Details of NWs grown on different orientations of sapphire

4. Optical characterization

The optical properties of the GaN NWs were studied using photolumi-nescence (PL) spectroscopy from ensembles of NWs. The sample was excitedwith a 266 nm frequency-quadrupled Nd:YAG laser. The light emitted by thesample was dispersed through a 0.55 m monochromator and detected witha thermoelectrically cooled Si-CCD. The PL spectra of GaN NWs at 10 K

300 350 400 450 500 550 600

PL c

ount

s (a

rb. u

nits

)

Wavelength (nm)

c-sapphire r-sapphire m-sapphire

Energy (eV)4 3.5 3 2.5

Figure 5: PL from GaN NWs: PL spectra of ensembles of NWs at 10 K that weregrown on c-plane, r-plane and m-plane sapphire substrates and mechanically transferredto an Si/SiO2 substrate.

grown on c-plane, r-plane and m-plane sapphire are shown in figure 5(a). Thespectra are normalised such that the near-band-edge emission peak is of thesame intensity. PL spectra from these NWs peak around 3.50 eV (354 nm),which corresponds to the near-band-edge emission of GaN [27, 24, 28]. TheFWHM of the PL peaks are ∼ 22 nm (220 meV), ∼ 42 nm (390 meV) and∼ 37 nm (350 meV) for NWs grown on c-plane, r-plane and m-plane sap-phire respectively. The width of these spectra could be due to the variationin thickness and properties of the different wires of the ensemble [24], or

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due to defect related luminescence [29]. An additional peak is observed at∼ 3.1 eV (400 nm) which might be due to strongly localized excitons [29].Yellow luminescence from these samples is relatively small indicating goodcrystal quality.

5. Raman spectroscopy of NWs

Raman spectroscopy was used to study the phonon modes of GaN NWs.A Witec alpha 300R confocal Raman microscope was used along with a532 nm frequency doubled Nd:YAG laser for excitation. Figure 6 shows the

2 0 0 4 0 0 6 0 0 8 0 00

5 0

1 0 0

1 5 0

E 1 ( L O )

A c . O v .A 1

P a r a l l e l t o E i n P e r p e n d i c u l a r t o E i n

Inten

sity (c

ounts

)

R a m a n s h i f t ( c m - 1 )

A c . O v .A 1 , E 2

E 2 ( h i g h )

Figure 6: Raman spectrum of a NW grown on c-plane sapphire

Raman spectrum for a GaN NW, that was transferred to an aluminium foilwhich serves as the substrate. The incident electric field (Ein) is perpendic-ular to the NW axis. The polarisation of the scattered light (Es) is collectedis either parallel to incident electric field (i.e perpendicular to the NW axis)and perpendicular to the incident electric field (i.e parallel to the NW axis).(The spectra shown is after background correction.) The peak at ∼ 308 cm−1

is seen when Es is parallel to Ein. It arises from the acoustic overtone withA1 symmetry corresponding to the H-point in the Brillouin zone. The peakobserved in the spectra with Es is perpendicular to Ein at ∼ 418 cm−1 alsocorresponds to an acoustic phonon overtone.[30, 31]. At ∼ 567 and 734 cm−1

the E2 (High) and E1 (LO) are seen.

6. Conclusions

In conclusion, we have grown GaN NWs on c-plane, m-plane, and r-planesapphire substrates using nickel catalyst and TMGa and NH3 as precursorsby MOCVD. A pressure of 150 torr in N2 environment and ∼ 840 ◦C, with

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low precursor flow rates and V-III ratio, was optimum for growing thin, non-tapering GaN NWs. The wires had a triangular cross-section and grew alongthe <1011> and <1010> directions. Low temperature PL shows near-band-edge emission from the NWs and small yellow luminescence indicates goodcrystalline quality. Raman spectroscopy reveals the good crystal quality ofthese NWs.

7. Acknowledgments

The authors are thankful to Priti Gupta, Sandip Ghosh, Ritam Sinhaand S.C. Purandare at TIFR, India and Subramaniyam Nagarajan at AaltoUniversity, Finland for support in materials characterization and useful dis-cussions. This work at TIFR was supported through internal grants 12P0168and 12P0169. C.B.M. acknowledges travel support from the Department ofScience and Technology, India, through the collaborative project INT/Finland/P-10.

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