Abstract
We study numerically the phonon dispersion relations and corresponding displacement fields for a circular crosssection nanowire superlattices consisting of anisotropic GaN and AlN. We determine a set of parameters which gives complete phononic bandgaps. The results suggest the potential for manipulating phonons in the micro/nano electromechanical systems.
Keywords:
Nanowire superlattice; Acoustic phonon; Phononic crystal; Phononic bandgapBackground
Phononic crystals (PCs) are composite materials made of arrays of constituents embedded in host materials [13]. The interesting characteristics of the PCs are related to the existence of phononic bandgaps (i.e., frequency gaps) due to the Bragg reflections of the phonons with long wavelengths. We can regard the PC as an opaque barrier for the phonons within the phononic bandgaps [4]. This suggests the potential for designing various phonon optic devices, such as phonon filters, mirrors, resonators, etc.
Recent advances in fabrication methods enable realization of onedimensional heterostructures, i.e., nanowire superlattices (NWSLs) [510]. Their electronic and optical properties were studied, and a variety of possible applications utilizing the characteristics were also proposed [1114]. In addition, the NWSLs are expected to yield interesting physical effects on phonons, which influence the electronic states and the transport properties via the electronphonon interaction. These NWSLs can be regarded as wiretype phononic crystals (WPCs), in which the phononic bandgaps are induced by the periodicity along the wire axis.
In a previous paper [15], we developed a numerical method to derive phonon modes in a freestanding NWSL of anisotropic material with an arbitrary shape of crosssection. As examples, the phonon modes were calculated for the rectangular and square crosssection GaAs/AlAs and InP/InAs NWSLs composed of anisotropic materials [15,16].
Though above result revealed the important aspects of phonon modes in the NWSLs, it seems to be difficult to design WPCs with complete phononic bandgaps because in the dispersion relations of these NWSLs, many subbands are folded into the miniBrillouin zone and the frequencies of gaps are different with phonon modes. In addition, the gap widths are narrow in these NWSLs because the difference of the acoustic impedance between the GaAs and AlAs layers (or between the InP and InAs layers) is small. The NWSLs with large acoustic mismatch would be suitable for designing the phonon optic devices.
In the present work, we numerically calculate the dispersion relations and corresponding displacement fields for a circular crosssection NWSL consisting of GaN and AlN, and we determine a set of parameters which gives complete frequency gaps.
Methods
The equation giving the eigenfrequencies of phonon modes in a freestanding NWSL composed of anisotropic crystals was formulated in [15]. In this method, the displacement components u_{i} (i = x,y,z) are expanded in terms of a set of basis functions ϕ_{α} (r)
The expansion coefficients A_{αi} and the eigenfrequencies ω are determined by solving the generalized eigenvalue equation:
For the NWSLs composed of cubic materials, the matrix elements H_{βi,αℓ} and S_{βi,αℓ} are written as
where
Here, C_{μν}and ρ are the stiffness tensor and mass density, respectively, which are dependent on r in the NWSLs.
As basis functions, we adopt the product of powers of the Cartesian coordinates in the xy plane. The z dependence is expressed in the form of the Bloch wave:
Here, R denotes the radius of the wire; G is the reciprocal lattice vector; and V = ΠR^{2}D is the volume of the unit cell, where D is the length of the unit cell in the z direction. The basis functions are specified with α = (m,n,G). The expressions for the matrix elements H_{βi,αℓ}and S_{βi,αℓ} can be analytically obtained for the circular NWSLs.
Based on group theory [17,18], the phonon modes are classified symmetrically. In the present study, the constituent layers are assumed to be cubic materials, i.e., zincblende structure. The group of k is C_{4v} for 0 < k < Π/D. The irreducible representations are A_{1}, A_{2}, B_{1}, B_{2}, and E [15].
By considering the above symmetry, the symmetryadapted basis function in the present system can be constructed, and the phonon dispersion relations of each mode are independently calculated.
Results and discussion
The boundary condition at the free surface of the wire requires that the wave numbers in the lateral direction are discretized. On the other hand, the wave number k in the longitudinal direction has a continuous value. Therefore, even the homogeneous plain nanowire has subband structure. In the NWSL, the subbands are folded into the miniBrillouin zone. In other words, the size of the miniBrillouin zone and corresponding phononic bandgaps are determined by the periodicity D of the NWSL.
For the nanowires with smaller R, the maximum wavelengths in the lateral directions become shorter. As a result, the subbands except for the lowest dispersion curve of each mode go up to the higher frequency region, though a lot of subbands are folded into the miniBrillouin zone. Changing the ratio of R and D, we can control the phonon modes in the lower frequency range.
Figure 1a illustrates the phonon dispersion relations calculated for R = 5.0 nm and D = 20.0 nm (thicknesses of both GaN and AlN layers are 10.0 nm). Other parameters we used are as follows: ρ= 6.15 g/cm^{3}, C_{11} = 2.96, C_{12} = 1.54, and C_{44} = 2.06 (all in units of 10^{12} dyn/cm^{2}) for GaN; ρ = 3.26 g/cm^{3}, C_{11} = 3.04, C_{12} = 1.52, and C_{44} = 1.99 (all in units of 10^{12} dyn/cm^{2}) for AlN [19].
Figure 1. Phonon dispersion relations of the circular crosssection GaN/AlN NWSL. (a) R = 5.0 nm and D = 20.0 nm; (b) R = 5.0 nm and D = 45.0 nm (thicknesses of GaN and AlN layers are 25.0 and 20.0 nm, respectively). The B_{2} modes exist in higher frequency range, and all the E modes are doubly degenerated.
In the present frequency range, we can see four different modes, i.e., the A_{1}, A_{2}, B_{1}, and E modes. The dispersion curves corresponding to the B_{2} modes exist in higher frequency range. The lowest dispersion curve of the A_{2} mode is mostly overlapped with that of the A_{1} mode.
The complete frequency gaps are realized for the parameters we selected. For comparison, we show in Figure 1b the phonon dispersion relations calculated for R = 5.0 nm and D = 45.0 nm (thicknesses of GaN and AlN layers are 25.0 and 20.0 nm, respectively). In this example, the broader bandgaps of the A_{1}, A_{2}, and E modes are nearly coincident with each other. Here, we note that the complete bandgaps disappear if the isotropic approximation is used for each constituent layer.
The lowest dispersion curves of the A_{1} and A_{2} modes are linear in k, i.e., ω vanishes at k = 0. On the other hand, the lowest frequencies of the B_{1} and B_{2} modes have finite values. For thin (thick) NWSLs, the lowest frequencies of B_{1} and B_{2} modes become higher (lower).
Figure 2a,b shows the displacement patterns corresponding to the lowest A_{1} and A_{2} modes at k = Π / D, respectively. The lowest A_{1} mode has the feature of a dilatational mode, while the lowest A_{2} mode shows the feature of a torsional mode.
Figure 2. Displacement field patterns corresponding to the lowest (a) A_{1}, (b) A_{2}, (c) B_{1}, (d) B_{2}, and (e) E modes atk=Π/D.
Figure 2c,d,e shows the displacement patterns corresponding to the B_{1}, B_{2}, and E modes at k = Π / D, respectively. The lowest B_{1} mode shows the feature of a stretching mode, i.e., the alternating dilatation and contraction in the x and y directions, while the lowest B_{2} mode shows the features of a shear mode, i.e., alternating stretching in the two diagonal directions.
All dispersion curves of the E mode are doubly degenerate because the irreducible representation of the E mode is twodimensional. For the E modes, the lowest dispersion curve near k = 0 is proportional to k^{2}. This parabolic behavior is due to the fact that these modes correspond to the bending of the NWSL. Figure 2e clearly shows that the E modes have a feature of flexural mode.
Conclusion
We theoretically studied the acoustic phonon modes in circular crosssection NWSLs consisting of cubic GaN and AlN. We calculated their dispersion relations and phonon displacement fields. These modes are classified into five types, i.e., A_{1}, A_{2}, B_{1}, B_{2}, and E modes, which have features of dilatational, torsional, stretching, shear, and flexural modes, respectively. We determined a set of parameters which gives complete phononic bandgaps. The results suggest the realization of the optimized phonon devices, such as phonon filters or mirrors in the micro/nano electromechanical systems.
In the present work, we only showed the results for the NWSLs consisting of cubic materials (i.e., zincblende structure). The results for the NWSLs with wurtzite structure will be given and the difference will be discussed elsewhere.
Competing interests
The author declares that he has no competing interests.
Acknowledgements
The author would like to thank N. Nishiguchi and Y. Nakamura for their useful discussions. This work was partially supported by a GrantinAid for Scientific Research (grant no.: 21560002) from the Japan Society for the Promotion of Science (JSPS).
References

Phys Stat Sol (b) 2004, 241:3454. Publisher Full Text

Vasseur JO, DjafariRouhani B, Dobrzynski L, Kushwaha MS, Halevi P: Complete acoustic band gaps in periodic fibre reinforced composite materials: the carbon/epoxy composite and some metallic systems.

Vasseur JO, Deymier PA, DjafariRouhani B, Pennec Y, HladkyHennion AC: Absolute forbidden bands and waveguiding in twodimensional phononic crystal plates.

Mizuno S: Eigenfrequency and decay factor of the localized phonon in a superlattice with a defect layer.

Gudikson MS, Lauhon LJ, Wang J, Smith DC, Lieber CM: Growth of nanowire superlattice structures for nanoscale photonics and electronics.
Nature(London) 2002, 415:617. Publisher Full Text

Wu Y, Fan R, Yang P: Blockbyblock growth of singlecrystalline Si/SiGe superlattice nanowires.
Nano Lett 2002, 2:83. Publisher Full Text

Bjork MT, Ohlosson BJ, Sass T, Persson AI, Thelander C, Magnusson MH, Deppert K, Wallenberg LR, Samuelson L: Onedimensional steeplechase for electrons realized.
Nano Lett 2002, 2:87. Publisher Full Text

Solanki R, Huo J, Freeouf JL, Miner B: Atomic layer deposition of ZnSe/CdSe superlattice nanowires.
Appl Phys Lett 2002, 81:3864. Publisher Full Text

Lassen B, Willatzen M: Transport through an array of quantum dots using modified Wannier states.

Zhang A, Voon LCLY, Willatzen M: Dynamics of a nanowire superlattice in an ac electric field.

Bjork MT, Ohlosson BJ, Thelander C, Persson AI, Deppert K, Wallenberg LR, Samuelson L: Nanowire resonant tunneling diodes.
Appl Phys Lett 2002, 81:4458. Publisher Full Text

Thelander C, Martensson T, Bjork MT, Ohlosson BJ, Larsson MW, Wallenberg LR, Samuelson L: Singleelectron transistors in heterostructure nanowires.

Kikuchi A, Kawai M, Tada M, Kishino K: InGaN/GaN multiple quantum disk nanocolumn lightemitting diodes grown on (111) Si substrate.
Jpn J Appl Phys 2004, 43:L1524. Publisher Full Text

Sekine T, Suzuki S, Kuroe H, Tada M, Kikuchi A, Kishino K: Raman scattering in GaN nanocolumns and GaN/AlN multiple quantum disk nanocolumns.

Mizuno S, Nishiguchi N: Acoustic phonon modes and dispersion relations of nanowire superlattices.
J Phys: Condens Matter 2009, 21:195303. Publisher Full Text

Mizuno S, Nakamura Y: Vibrational modes in a square crosssection InAs/InP nanowire superlattice.
Phys Status Solidi C 2010, 7:370. Publisher Full Text

Inui T, Tanabe Y, Onodera Y: Group Theory of Crystal Lattice and Its Applications in Physics. SpringerVerlag, Berlin; 1996.

Bradley CJ, Cracknell AP: The Mathematical Theory of Symmetry in Solids. Clarendon Press, Oxford; 1972.

Vurgaftman1 I, Meyer JR, RamMohan LR: Band parameters for IIIV compound semiconductors and their alloys.
J Appl Phys 2001, 89:5815. Publisher Full Text