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Research ArticleReceived: 22 November 2009 Accepted: 15 April 2010 Published online in Wiley Online Library: 16 June 2010

(wileyonlinelibrary.com) DOI 10.1002/jrs.2704

Surface optical Raman modes in GaNnanoribbonsSantanu Bhattacharya,a Anindya Datta,a Sandip Dharab∗and Dipankar Chakravortya∗

Raman scattering studies were performed in GaN nanoribbons grown along [1 0 0]. These samples were prepared inside Na-4mica nanochannels by the ion-exchange technique and subsequent annealing in NH3 ambient. Detailed morphological andstructural studies including the crystalline orientation were performed by analyzing the vibrational properties in these GaNnanoribbons. Pressure in the embedded structure was calculated from the blue shift of the E2(high) phonon mode of GaN.Possible red shift of optical phonon modes due to the quantum confinement is also discussed. In addition to the opticalphonons allowed by symmetry, two additional Raman peaks were also observed at ∼633 and 678 cm−1 for these nanoribbons.Calculations for the wavenumbers of the surface optical (SO) phonon modes in GaN in Na-4 mica yielded values close tothose of the new Raman modes. The SO phonon modes were calculated in the slab (applicable to belt-like nanoribbon) mode,as the wavenumber and intensity of these modes depend on the size and the shape of the nanostructures. The effect ofsurface-modulation-assisted electron–SO phonon scattering is suggested to be responsible for the pronounced appearance ofSO phonon modes. A scaling factor is also estimated for the interacting surface potential influencing the observed SO Ramanscattering intensities. Copyright c© 2010 John Wiley & Sons, Ltd.

Supporting information may be found in the online version of this article.

Keywords: surface optical mode; surface potential; Raman scattering; GaN; Na-4 mica; nanobelt

Introduction

It is well known that GaN is a technologically important materialfor optoelectronics, e.g. in the fabrication of blue light-emittingdiodes and laser diodes,[1] and has potential applications inhigh-temperature and high-power electronics.[2] GaN has beenextensively studied in the form of thin films,[3] one-dimensional(1D) nanowires,[4] two-dimensional (2D) nanobelts,[5] and bulkcrystal.[6] In the heteroepitaxial method of 2D growth, a very largevariation of lattice constant and thermal conductivity betweenthe substrate and the film causes the formation of defects. Thesituation can be improved by introducing a homoepitaxial GaNbuffer layer, and adopting the stress removal technique of forminga grating in this buffer layer.[1] The stress removal is performedmost successfully by lateral epitaxial growth with a dislocationdensity of ∼107 cm−2.[7] Recently, we have reported the growthof ultrathin GaN films with low defect density embedded in aspecially prepared Na-4 mica template, which show unusuallylong excitonic lifetime.[8]

Among other techniques, the Raman scattering is also used toprobe the surface optical (SO) phonon modes of nanostructures.[9]

Parameters such as defects, size distribution, and coupling withthe surrounding medium may add to nanodimensionality indefining the Raman signature.[10] For nanocrystals, lattice vibrationis confined to the surface, giving rise to a wavenumber in betweenthose of the transverse optical (TO) and longitudinal optical (LO)phonon modes.[10,11] However, the SO phonon modes cannotbe observed for a perfect surface, because of the constraintsin momentum conservation. Surface roughness or constructionof a grating capable of absorbing the phonon momentum is anecessary condition for perturbing the surface potential in order

to make the SO phonon observable.[12] There are very few reportsavailable on the Raman study of SO phonons in nanostructuredIII–V nitride systems. As a matter of fact, there is no experimentalstudy of SO modes available in AlN. In a recent study, Hsiaoet al.[13] reported on an SO phonon at 708.7 cm−1 in a single 1DGaN nanowire, as a broad, hump-like feature. In addition, a GaNnanopillar of diameter ∼50 nm is reported to show an SO phononmode at ∼716 cm−1.[14] We also reported SO phonon modes at528 and 560 cm−1 for InN nanostructures,[15] and an SO mode at∼680 cm−1 for the In0.15Ga0.85N/GaN multi-quantumwell.[16]

In the present study, the fabrication of GaN nanoribbonsembedded in Na-4 mica with around 2–3 monolayer thickness isachieved by an ion-exchange process with subsequent nitridationtreatment. We report here the identification of the SO phononmodes in the GaN nanoribbons along with calculations, takinginto account the surrounding dielectric medium.

∗ Correspondence to: Dipankar Chakravorty, DST Unit on Nanoscience and MLSUnit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata700032, India. E-mail: mlsdc@iacs.res.in

Sandip Dhara, Surface and Nanoscience Division, Indira Gandhi Centre forAtomic Research, Kalpakkam 603102, India. E-mail: dhara@igcar.gov.in

a DST Unit on Nanoscience and MLS Unit, Indian Association for the Cultivationof Science, Jadavpur, Kolkata 700032, India

b Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research,Kalpakkam 603102, India

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Figure 1. (a) A GaN straight nanoribbon. Arrows are shown for theobserved surface modulations. (b) A lattice image of a nanoribbon withthe presence of crystalline planes of wurtzite phase and growth directionalong [1 0 0] in the basal plane. The inset shows the corresponding SAEDpattern with the zone axis lying along [0 0 1]. (c) A stack with numerousnanoribbons (side view) sandwiched in mica.

Experimental

The mica of composition Na4Mg6Al4Si4O20F4 · xH2O, commonlyreferred to as Na-4 mica, was synthesized using the sol–gelmethod. The precursors used were Al(NO3)3 9H2O, Mg(NO3)2·6H2O, and tetraethylorthosilicate (TEOS, Si(OC2H5)4). The detailsof the synthesis are given elsewhere.[17] The Na-4 mica powderwas kept inside a solution of Ga(NO3)3 in water for ∼2 months atroom temperature (RT) to bring about an ion-exchange reaction3Na+ ⇔ Ga3+. Also, the nanochannels were filled with Ga(NO3)3

molecules which diffused into the channels. The powder was thenthoroughly washed and dried several times. Finally, the resultingpowder was subjected to a nitridation treatment. The powderin an alumina boat was brought to 1323 K under nitrogen flowinside a tubular furnace and then subjected to NH3 gas flow atthat temperature for 1 h. The powder was brought back to RTby cooling the furnace under nitrogen flow. The GaN phase wasformed with the decomposition of NH3 in the reactor chamber.A high-resolution transmission electron microscope (HRTEM, Jeol2010) was used at 200 kV for studying detailed microstructures,and selected-area electron diffraction (SAED) pattern includinglattice image was used for structural studies. Details of the samplepreparation for TEM analysis will be reported elsewhere.[8] Micro-Raman spectroscopic measurements (objective magnification100×) were performed using the 325-nm excitation of a 30mW diode-pumped He–Cd laser and analyzed with a Jobin-Yvon HR800 spectrometer fitted with 1800 lines/mm gratings(spectral resolution ∼0.5 cm−1). The system is equipped with aliquid-nitrogen-cooled CCD detector for recording the spectra.

Results and Discussion

Structural studies

The GaN sample, extracted from the Na-4 mica nanochannelsby etching,[8] was investigated with HRTEM. Figure 1(a) shows atypical image of one such GaN nanoribbon, and Fig. 1(b) showsthe high-resolution lattice image taken from a nanoribbon withmarked crystallographic planes (1 1 2 0), (1 −2 −1 0), and (2 −1 1 0)of wurtzite phase. Crystalline orientation in the basal plane isalso marked with growth orientation along [1 0 0]. The inset in

Fig. 1(b) shows the SAED pattern. Analyses of lattice image andassociated diffraction pattern of wurtzite GaN showed that thezone axis lay along [0 0 1]. In the side view, a stack with a largenumber of GaN nanoribbons (∼50–60 nm width) sandwichedbetween mica is also shown (Fig. 1(c)). On a larger scale, powderX-ray diffraction studies also showed excellent crystallinity ofthese samples (Fig. S1, Supporting Information). During the ion-exchange process, Ga3+ ions encounter an environment similarto that of the Na+ ions, which are loosely connected to the restof the structure of Na-4 mica. During the growth of GaN afternitridation, the stacking layers of mica on both sides probablydo not play the usual role of substrate as observed in homo- orheteroepitaxy. They rather provide a confined space of ∼0.7 nmfor the growth of 2D GaN films, which is barely wide enough tocreate just a few layers of GaN (c = 0.5178 nm for GaN; about 2–3monolayers of GaN in these nanochannels considering thermallyexpanded situation). This claim is substantiated with the thicknesscalculated when applying a quantum confinement model to theexperimental shift in the photoluminescence peak position at RT(Fig. S2 (Supporting Information) and associated comments),[8]

as also reported for 0.6–1.5-nm GaN quantum wells in GaN/AlNmultilayer structures.[18]

UV Raman scattering studies

The hexagonal GaN crystallizes in the wurtzite structure belongingto the c4

6ν (P63mc) space group. There are two formula units perprimitive cell and all the atoms occupy the sites of symmetry C3v .Using a conventional group theory analysis, it can be shown thatsix optical modes, i.e. A1(TO) + A1(LO) + E1(TO) + E1(LO) + 2E2, canbe observed in the first-order Raman scattering.[19] Fig. 2(a) showsa typical Raman spectrum obtained for the GaN layers grown insidethe nanochannels of Na-4 mica. The large background is obviouslydue to the large contribution from the Na-4 mica (inset in Fig. 2(a)).Raman peaks at ∼321 and 421 cm−1 were reported as acousticovertones for wurtzite GaN.[20] Acoustic modes are reported fornanostructures where translational symmetries are broken as aneffect of finite crystal size. The peak at 450 cm−1 may correspondto the 432 cm−1 peak of pristine Na-4 mica (inset of Fig. 2(a)). Thewavenumber difference may be due to the annealing effect in theNa-4 mica sample, which might have modified the ion-exchangeprocess. The peaks at higher wavenumbers are fitted separatelywith Lorentzian line shapes (Fig. 2(b)). The peaks at ∼560, 577,and 733 cm−1 correspond to the E1(TO), E2(high), and A1(LO)symmetries, respectively.[21,22] Except for a blue shift of ∼8 cm−1

in the peak position of E2(high) mode, there is no significantshift for the other two modes. With the known hydrostaticpressure coefficient (Raman wavenumber shift with unit pressure)for E2(high) mode (4.17 cm−1 GPa−1),[23] we calculate a pressureof ∼2 GPa in these samples. A similar blue shift of ∼7 cm−1 of theobserved mode wavenumber was also expected for the E1(TO)(pressure coefficient = 3.68 cm−1 GPa−1)[23] and A1(LO) (pressurecoefficient = 3.2 cm−1 GPa−1)[24] modes, which might have beencompensated with the red shift due to the phonon confinementeffect. The red shift may be accounted for by the contributionof optical phonon frequencies away from zone center in caseof broken translational symmetry,[25] and the dispersion relationof E1(TO) and A1(LO) modes in GaN.[26] As a matter of fact, ared shift of 9 cm−1 from the reported peak corresponding tobulk A1(LO)[21,22] mode at 735 cm−1 is observed. The line shapecalculated for A1(LO) mode confined in a GaN nanoribbon with athickness of 1.2 nm (close to what is found in our samples) peaks

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Figure 2. (a) Raman spectra of Na-4 mica–GaN with excitation at 325 nm.The inset shows the Raman spectra of as prepared Na-4 mica. (b) Peakscorresponding to the higher wavenumbers fitted (continuous line forthe spectrum and dashed lines for the individual peaks) with Lorentzianlineshapes corresponding to various known modes of GaN along with SOmodes.

at 726 cm−1. The peak measured at 733 cm−1 is much closer to thereference 735 cm−1 wavenumber in bulk GaN. The compensationof the size-confinement effect by a stress-induced blue shift isa possible explanation. In this context, we must state that theobserved shift may also be due to the shift of the peak position forthe optical phonon modes of A1 and E1 symmetries depending onthe relative sample-to-laser orientation.[27] However, prominentpeaks at ∼633 and 678 cm−1 cannot be accounted for by thereported Raman modes in GaN. Thus, we examine the possibilityof surface phonons being responsible for the new modes foundfor GaN.

Calculation of SO modes

In polar crystals, SO modes are observed on the low wavenumberside of the LO mode.[10,11] Large intensities in SO phonons havebeen reported in polar semiconductor nanowires[9,28] as well asionic nitride semiconductor nanostructures.[13 – 16] High intensitiesof the SO modes for the large surface area available to thenanostructure may be quite useful in detecting small oscillationsof surface profile in the reduced dimension. Owing to surfaceroughness and even surface modulation of GaN nanoribbons’cross-section (Fig. 1(a)),[9,28] we anticipate that these unidentified

Figure 3. SO phonon dispersion curves calculated for Na-4 mica encapsu-lated GaN nanoribbons assuming rectangular cross-section (Eqns (1) and(2)). The continuous and dashed-dot-dot curves correspond to E1 andA1 symmetries, respectively. AS and S modes are indicated in the plot.Two dashed horizontal lines represent the experimentally measured SOwavenumbers at 633 and 678 cm−1. Four other horizontal dotted linesrepresent the LO and TO wavenumbers of bulk GaN corresponding to theE1 and A1 symmetries. From the intersection between the experimentallymeasured wavenumbers and the corresponding dispersion (marked byvertical dashed dotted lines), we can predict the important wavelength ofthe perturbation potential that breaks the symmetry and activates the SOmodes.

peaks may originate from the SO modes. The SO phononwavenumber and intensity mainly depend on the size and theshape of the nanostructured material. The dispersion relations forSO phonons (ωSO) for a nanoribbon (belt like) are given by[9,11]

ω2SO(q)S = ω2

TO

[ε0 tanh(qiLi/2) + εm

ε∞ tanh(qiLi/2) + εm

](1)

ω2SO(q)AS = ω2

TO

[ε0 coth(qiLi/2) + εm

ε∞ coth(qiLi/2) + εm

](2)

where ωTO is the wavenumber of the TO phonon. Equation (1)gives the symmetric (S) SO mode wavenumber, whereas Eqn (2)refers to the antisymmetric (AS) SO mode. ε0 and ε∞ are the low-and high-frequency dielectric constants of the material; εm is thedielectric constant of the medium surrounding the slab; Li(i = x, y)is the edge width of the rectangular slab whose growth directionis along z; and qi(i = x, y) is the phonon wavevector. In order toobtain the dispersion relations (Eqns (1) and (2)) for the SO mode,consideration goes with similar parity for the potentials of TO andLO optical phonon modes in the x and y directions (q2

x + q2y = q2;

qx Lx = qy Ly)[29] and the estimation of dielectric constants ofthe medium following the Lyddane–Sachs–Teller (LST) relationcorresponding to A1 and E1 symmetries.[30] As mentioned earlier,the nanoribbons are grown along the a axis [1 0 0] (Fig. 1(b)). Takinga value of 10.4 for ε

[100]0 , we can calculate ε[100]∞ = 5.47 and 5.93

corresponding to A1 and E1 symmetries, respectively, followingthe LST relation.[31] We have used bulk values ωLO = 735 cm−1

and ωTO = 533 cm−1 for A1 symmetry and ωLO = 743 cm−1

and ωTO = 561 cm−1 for E1 symmetry.[21,22] We have derived thedispersion relation ωSO(q) for a GaN nanoribbon encapsulated inNa-4 mica (εm = 17).[32] Fig. 3 shows a plot of both the S and ASmodes of SO phonon dispersion curves for A1 and E1 symmetries.

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Table 1. Estimation of the surface potential perturbation wave-lengths (λ) activating SO modes

λ = πL/q (nm)

L (nm)q = 0.16

(ωSO = 678 cm−1)q = 0.63

(ωSO = 633 cm−1)

1 20 5

50 1000 250

Horizontal lines corresponding to the experimentally observedwavenumbers at 633 and 678 cm−1 only intersect the dispersioncurves of AS SO modes at qiLi/2 values, which are marked withvertical lines. The wavevectors q of the surface perturbation thatis responsible for activating the SO modes are estimated with theexperimental 633 and 678 cm−1 ωSO values (Fig. 2(b)). We findthat qiLi/2 = 0.16 and 0.63 correspond to A1 and E1 symmetries,respectively. In the case of nanostructures, or for that matter on thesurface, both even and odd modes of polarization eigen vectorsare allowed for the breakdown in translational symmetry. Thus,both AS and S modes can respond to SO phonon modes. In thepresent study, we find the intersection only with AS mode, whichis only incidental.

One can also see that the intensities of the observed SOphonons in the present study are comparable to those of theother phonons. The magnitude of surface roughness determinesthe SO peak intensity owing to the breakdown of the translationalsymmetry of the surface potential.[9,28] If we take the averagenanoribbon cross-section dimensions to be 50 × 1 (Lx × Ly)nm2, we can then calculate the wavelengths for the surfacepotential perturbation that breaks the symmetry and activatesthe SO mode Raman scattering (Table 1), i.e. λ = 2π/qi (i = x, y)20–1000 nm (A1 symmetry) and 5–250 nm (E1 symmetry). Surfaceroughness with a modulation of ∼50 nm is observed (indicatedby arrows in Fig. 1(a)), and any integral multiple equivalent tothe λ (5 × 50 nm = 250 nm or 20 × 50 nm = 1000 nm) will besufficient to initiate breakdown of translational symmetry for thecontribution of surface potential to the SO modes. The interactionpotentials for the SO modes scales as (qzLx)−1 and (qz Ly)−1

resulting in the electron–SO phonon scattering in nanoribbonsdominating over electron–LO confined phonon scattering rate forqz � L−1

x (L−1y ).[29] In the present study, qz ≈ 2×105−6×104 cm−1

(λ = 250–1000 nm) � L−1y ≈ 1 × 107 cm−1 (Ly ≈ 1 nm), which

makes the intensity of the surface mode comparable to that of theother optical modes. We must state here that the dispersion in thedistribution of dimensions in these samples is relatively narrowas shown in Fig. 1(c), about 50 nm. However, other rigid modescorresponding to even L (quantum number corresponding to axialmodes) in the continuum model of the elongated nanostructurescontribute to the width of the SO peak.[33] Strong coupling ofelectron–SO phonon may be useful for molecular detection withdielectric contrast in the nanostructures.

Conclusion

In summary, we have synthesized 2D GaN nanoribbons made ofabout 2–3 monolayers using Na-4 mica nanochannels as template.The first-order Raman spectra of this system are used to identifythe vibrational modes of this encapsulated GaN ultrathin 2Dsystem in Na-4 mica. The ultrathin nanoribbons show pronounced

SO phonon modes because of surface modulation or roughness.The results are supported with HRTEM. A strong electron–SOphonon coupling for wavevectors smaller than the inverse of thenanoribbon cross-section is suggested to be responsible for theenhanced intensity of SO modes.

Acknowledgements

This work was supported by DST, New Delhi, through a grantunder its Nano mission. SB thanks CSIR, New Delhi, for theaward of a Senior Research Fellowship. DC thanks Indian NationalScience Academy, New Delhi, for awarding him an HonoraryScientist’s position. The authors thank Basab Chattopadhyay forhelp with some experiments. They also thank S. Sahoo of theCondensed Matter Physics Division, IGCAR, Kalpakkam, India, forthe calculation of phonon confinement. SD thanks Dr Baldev Rajfor his constant encouragement.

Supporting information

Supporting information may be found in the online version of thisarticle.

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