Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

Cheng-Ying Chen; Jun-Han Huang; Kun-Yu Lai; Yi-Jun Jen; Chuan-Pu Liu; Jr-Hau He

doi:10.1364/OE.20.002015

Due to their large aspect ratio, semiconducting one-dimensional (1D) nanostructures are promising elements for realizing fascinating optoelectronic devices, such as photodiodes [1,2], photodetectors [3–7], solar cells [8], light-harvesting layers [9], nanolasers [10], and light-emitting diodes (LEDs) [2,11]. Moreover, it was found that the single nanowire/nanorod (NW/NR) with subwavelength diameter, large aspect ratio, and high permittivity exhibits a strong optical anisotropy. For example, the giant polarization anisotropy of light emission/absorption has been reported with individual InP NWs [12], Si NWs [13], CdSe NWs [14], GaN NRs [15], and ZnO NWs [16,17]. Owing to the optical polarization anisotropy, single or ensemble NWs/NRs can function as polarization sensitive photodectectors [12,18], compact polarization converters in optical communication [19,20], phase-matched nonlinear optical components [21,22], interferometer-based optical sensors [23], and propagation medium for surface waves [24]. For the ensembles of aligned 1D nanostructures, polarization anisotropy results in optical birefringence, which has been found in GaP NW arrays (NWAs) [25,26] and carbon nanotube arrays [27]. The strong artificial birefringence between the in-plane and out-of-plane refractive indices was also observed in the well-aligned GaP NWAs [25]. However, for practical applications, the in-plane birefringence of nanostructured films is desirable for a wide range of photonic applications [28].

The oblique-angle deposition (OAD) has been used to fabricate thin film materials that exhibit artificial birefringence [29–33]. Several attempts have been made to improve the birefringence of oblique angle-deposited thin films. For example, serial bideposition, a variation of the OAD, was utilized to greatly enhance in-plane birefringence [34]. Post-annealing was demonstrated to improve linear birefringence by the densification of the anisotropic columns [29]. Xiao et al. reported that the combination of the OAD and the sol–gel techniques can improve the linear birefringence of SiO₂ thin films [32]. Moreover, optoelectronic devices would greatly benefit from a thin film material whose in-plane birefringence is as large as possible while remaining single crystalline. However, the OAD nanostructured films show poor crystallinity and thus inefficient light emission ability [29,30,35,36], restricting their applications in passive photonic components, such as waveplates [35,37]and birefringent/transparent electrical conductors [31]. Additionally, OAD nanostructured films exhibit low in-plane birefringence in the range of 0.01–0.07 [31,35] due to a limited range of the column tilt angles (30°–55° relative to the substrate normal) [30,33,35].

In this study, a combined method of modified OAD and hydrothermal growth was utilized to obtain a novel photonic metamaterial based on single crystalline ZnO NWAs with oblique angles in the range of 75°–85°, exhibiting large artificial in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of the NWAs layer is almost one order of magnitude higher than that of bulk ZnO ( $Δ n ≅$ 0.015) [38,39]. The notable polarization degree in the photoluminescence (PL) emission of the NWAs is due to the optical confinement effect. The feasibility of single crystalline oblique-aligned NWA growth offers an excellent opportunity for the application in ensemble polarization-sensitive optical devices.

The oblique-aligned ZnO NWAs were grown on Si(100) substrate with a sputtering process and a subsequent hydrothermal method. Pure ZnO target (99.99%) was utilized as the sputtering source. First, a ZnO buffer layer was prepared by the two-steps sputtering process: (a) the ZnO layer was deposited on a 1-rpm-rotating substrate in argon at 410 °C at the oblique angle of 30° with respect to the surface normal of the sputtering target; (b) the ZnO bent columnar seed was deposited in a reduced atmosphere with 20% hydrogen/argon mixture gas at 265 °C at the oblique angle of 30° without the substrate rotation. For the final hydrothermal process, the oblique-aligned ZnO NWAs were synthesized in a solution mixed by 0.005 M Zinc acetate dehydrate (Zn(Ch₃COO)₂.2H₂O) and 0.005 M Hexamethylenetetramine (HMT C₆H₁₂N₄) at the ratio of 1:1, heated at 81 °C for 2 hours. More details of oblique-aligned NWA synthesis are described elsewhere [40].

Morphological studies were performed with a JEOL JSM-6500 field emission scanning electron microscopy (SEM) and a JEOL 3000F field emission transmission electron microscopy (TEM). Optical reflectance measurements were performed at the angle of incidence (AOI) of 5° for both s- and p-polarization in the wavelength ranges of 300–1400 nm by a standard UV-VIS-NIR spectrophotometer (JASCO V-670). The reflection of a collimated incident light beam was measured by collecting the specularly reflected cone of light within an acceptance angle of 6°. The PL measurements were performed in air at room temperature using a He-Cd laser (photon energy = ~3.8 eV) as an excitation source with 1.5 mm diameter of beam spot and 35 mW excitation power. The polarized PL measurements were performed by placing respectively an analyzer and a depolarizer into the path of the collected signal beam. The depolarizer was placed between a spectrometer and the analyzer in order to eliminate the polarization dependence of the measurement equipments.

Figure 1(a) shows the typical SEM images of the oblique-aligned ZnO NWAs. OAD results in the formation of bent columnar ZnO seed layers inclined from the substrate normal in the opposite direction to the incident vapor beam due to surface diffusion-enhanced self-shadowing effect, which is distinct from the NWA growth using conventional OAD methods [31,40]. The oblique-aligned ZnO NWAs were uniformly synthesized on the bent columnar ZnO seed layers using a solution method. One can see that the NWAs do not exhibit the broadening effects that are prevalent in conventional OAD nanostructured films. The diameter and the length of the NWs are in the ranges of 80–100 nm and 0.8–1.2 μm, respectively. The cross-sectional TEM image in Fig. 1(b) shows that the tilted angles of the NWs are in the range of 75°–85° relative to substrate normal. It has been found that in-plane birefringence of the nanostructured films increases in magnitude with oblique deposition angle [41]. Accordingly, the ultrahigh oblique angle of NWAs obtained here is a key to achieve giant optical anisotropy and hardly obtained by a conventional OAD method. The high-resolution TEM (HRTEM) image indicates that the NWs are single crystalline, as shown in Fig. 1(c). The measured interplanar distance of 0.26 nm corresponds to the ZnO(0002) planes, showing that the NWs grow preferentially along the c-axis direction. It is reported that preferential orientation along the ZnO [0001] direction has been observed in ZnO thin film growth on arbitrary substrates, indicating that the growth of textured ZnO seeds with their c-axes normal to the substrate by the decomposition of zinc acetate takes place inherently [42]. Accordingly, the bent columnar ZnO seed is not related to the crystallinity of the substrate due to the existence of the ZnO buffer between the columnar structure and the Si substrate.

Fig. 1 (a) Top-view SEM image of the oblique-aligned ZnO NWAs. (b) Cross-sectional TEM image of the ZnO NWAs. (c) HRTEM image of a ZnO NW.

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The natural structure of ZnO is wurtzite [space group = P6₃mc], which lacks of cubic symmetry, resulting in anisotropic optical properties, i.e., the existence of spontaneous polarization along the c-axis [43]. Furthermore, photonic metamaterials consisting of subwavelength-scale structures have shown the effective optical properties significantly different from those of their individual constituent materials [44,45]. In order to investigate the optical anisotropy of the oblique-aligned ZnO NWAs, the reflection spectroscopy was performed with the plane of incidence aligning to the orientation of the NWAs. In this measurement, the electric fields of s- and p-polarized light are perpendicular and parallel to the azimuthal direction of the long axis (c-axis) of oblique-aligned NWAs, respectively. The measured reflectivity spectra of the ZnO NWAs/ZnO buffer layer/Si substrate are shown in Fig. 2 . The nearly vanished reflectivity at the wavelengths shorter than 368 nm is due to the bandgap absorption of ZnO. For the wavelengths longer than 368 nm, strongly modulated reflectivity spectra (i.e., Fabry–Pérot oscillations) can be clearly observed. The modulated spectra are attributed to the multiple reflection at the three optically flat interfaces of air/NWs, NWs/ZnO buffer layer, and ZnO buffer layer/Si substrate. It is worth noting that the reflectivity spectra with the electric field parallel to the azimuthal direction of the long axis of NWAs is significantly different from that perpendicular to the azimuthal direction of the long axis, demonstrating an obvious optical anisotropy of the oblique-aligned ZnO NWAs; i.e., the morphological anisotropy of the oblique-aligned NWA structure results in a reduction of the in-plane symmetry of the optical properties from an isotropy to an in-plane uniaxial symmetry.

Fig. 2 Reflectivity spectra of the oblique-aligned ZnO NWAs for s- and p-polarization at the AOI of 5°. The plane of incidence is aligned to the orientation of NWs. The s- and p-polarization is perpendicular and parallel to the azimuthal direction of the long axis of oblique-aligned NWAs, respectively.

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In order to quantitatively analyze the optical anisotropy of the NWAs, we performed the calculation of the Fresnel reflection in multiple layers to extract the equivalent reflective indices $n (λ)$ [46–49]. Here the two-layer system is used to simulate the reflectivity spectra. The top layer is the oblique-aligned ZnO NWAs surrounded by air, and the bottom one is the ZnO buffer layer on a semi-infinite Si substrate. To simplify our calculations, we consider the reflectivity spectra in the non-absorbing wavelength range above 700 nm for ZnO. In other words, we only consider the real part of refractive indices and utilize Cauchy equation to describe the dispersion relationship of refractive indices [50]. As considering wavelength range above 700 nm, the scale of the oblique-aligned ZnO NWAs belongs to the subwavelegnth scale. Therefore, it is reasonable to treat the NWAs as an optical layer. Moreover, the Fabry–Pérot oscillations in the reflectivity spectra of Fig. 2 is an evidence that there is no significant scattering at the interface between air and the ZnO NWAs in the wavelength regions (700–1400 nm) [48]. The equivalent refractive index is polarization dependent denoted as $n_{⊥} (λ)$ (s-polarization) $n_{∥} (λ)$ (p- polarization). The reflection coefficient of the reflected light (r) at normal incidence can be described by the Fresnel equations and obtained by summing up an infinite series of partial reflected waves after multiple reflection from the interfaces of air/NWAs, NWAs/ZnO buffer layer, and ZnO buffer layer/Si substrate. The resulting reflectivity is [47,51,52]

R = {| r |}^{2} = {| \frac{r_{1} + r_{2}^{'} e^{i φ}}{1 - r_{1} r_{2}^{'} e^{i φ}} |}^{2}

where

φ

is the phase difference due to the optical path within the NWAs;

r_{1} = (1 - n^{NWAs}) / (1 + n^{NWAs})

is the reflection coefficient at the air/NWAs interface; and

r_{2}^{'} = (r_{2} + r_{3} e^{i ϕ}) / (1 - r_{2} r_{3} e^{i ϕ})

, where

ϕ

is the phase difference due to the optical path within the buffer layer,

r_{2} = (n^{NWAs} - n^{buffer}) / (n^{NWAs} + n^{buffer})

and

r_{3} = (n^{buffer} - n_{Si}) / (n^{buffer} + n_{Si})

are the reflection coefficients at the NWAs/ZnO buffer layer and ZnO buffer layer/Si substrate interfaces, respectively [

n^{NWAs}

(

n^{buffer}

) is the equivalent refractive index of the NWAs (the buffer layer) and

n_{Si}

is the refractive index of Si substrate]. In addition,

φ = 4 π n^{NWAs} d_{NWA s} / λ

and

ϕ = 4 π n^{buffer} d_{buffer} / λ

, where

d_{NWAs}

(

d_{buffer}

) is the thickness of the NWAs (the buffer layer). In this case, the reflectivity R is a function of

n^{NWAs}

,

n^{buffer}

,

d_{NWAs}

and

d_{buffer}

, where

d_{NWAs}

and

d_{buffer}

were determined by cross-sectional SEM image (not shown here).

To analyze $n^{NWAs} (λ)$ , we have to obtain $n^{buffer} (λ)$ first. The $n^{buffer} (λ)$ can be retraced by fitting the reflectivity spectra of the buffer layer/Si substrate sample without the NWAs. The reflectivity R’ of the one-layer system is presented theoretically based on Eq. (1) by substituting $r_{1} = (1 - n^{buffer}) / (1 + n^{buffer})$ , $r_{2}^{'} = (n^{buffer} - n_{Si}) / (n^{buffer} + n_{Si})$ , and $φ = 4 π n^{buffer} d_{buffer} / λ$ . The fitting result is shown in the inset of Fig. 3 . Noteworthily, the measured spectra of the buffer layer/Si substrate for s-/p-polarization are almost identical, demonstrating an in-plane optical isotropy on the buffer layer. Moreover, there is good agreement between the calculated and the experimental values. The $n^{buffer}$ exhibits normal dispersion, in which the equivalent refractive index decreases with the wavelength; $n^{buffer} (λ)$ goes from 1.970 to 1.914 as the wavelength varies from 700 nm to 1400 nm, indicating a perfect consistency with the previously reported results of ZnO thin films [53].

Fig. 3 Determination of the equivalent reflective index dispersion curve of the ZnO buffer layer. The inset is the comparison of simulated and experimental reflectivity spectra of the ZnO buffer layer on Si substrate.

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Based on the aforementioned procedure, the reflectivity R of the two-layer system is a function of $n^{NWAs}$ . The $n^{NWAs} (λ)$ is derived by fitting the reflectivity spectra of the NWAs/buffer layer/Si substrate sample for s-/p-polarization, as shown in the insets of Figs. 4(a) and 4(b). The excellent agreement between the simulations and the measurements demonstrates the validity of the two-layer system. Accordingly, for the polarization perpendicular to the azimuthal direction of the long axis of oblique-aligned NWAs, $n_{⊥} (λ)$ of the NWAs is from 1.271 to 1.266 as the wavelength varies from 700 nm to 1400 nm, as shown in Fig. 4(a); for the polarization parallel to the azimuthal direction of the long axis of oblique-aligned NWAs, $n_{∥} (λ)$ of the NWAs is from 1.381 to 1.361 in the same wavelength range, as shown in Fig. 4(b). The difference in $n_{⊥} (λ)$ and $n_{∥} (λ)$ results from a unique geometrical distribution of the structure mixed with air and oblique-aligned ZnO NWAs. As shown in Fig. 4(c), the in-plane birefringence parameter of the oblique-aligned NWAs can be determined by the definition [54]:

Fig. 4 Determination of the equivalent reflective index dispersion curve of the oblique-aligned ZnO NWAs for the polarization (a) perpendicular and (b) parallel to the azimuthal direction of the long axis of oblique-aligned NWAs. (c) The in-plane birefringence of the oblique-aligned ZnO NWAs. The insets of (a) and (b) are the comparison of simulated and experimental reflectivity spectra of the NWAs on the ZnO buffer layer/Si substrate.

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Δ n = n_{∥} (λ) - n_{⊥} (λ)

Amazingly, the birefringence of the oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of bulk ZnO ( $Δ n ≅$ 0.015) [38] and the natural birefringence of quartz ( $Δ n ≅$ 0.01) [28]. Furthermore, our in-plane birefringence is also comparable with that of the anisotropic-nanostructured silicon ( $Δ n ≅$ 0.115) [19]. However, our ZnO NWAs can be more easily combined in ZnO or GaN LED applications.

In order to further investigate the in-plane optical anisotropy of the NWAs, the polarization-dependent reñectivity at the AOI of 5° for the NWAs was characterized by rotating the polarization at the wavelength of 528 nm, as shown in Fig. 5 . When the polarization is parallel to the azimuthal direction of the long axis of oblique-aligned NWAs (i.e., the angle of polarization was 0°), the minimum reflectivity of ~5.5% is observed. The reflectivity is increased to the maximum value of ~10.5% at the polarization angle of 90°. A huge difference between the reflectivity extrema (3.8%) at 528 nm corroborates the high in-plane birefringence in our oblique-aligned NWAs.

Fig. 5 The reflectivity of the oblique-aligned ZnO NWAs characterized with the polarization angles between 0° and 90° at the wavelength of 528 nm and the AOI of 5°.

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The strong polarization anisotropy of the emission spectra has been observed in the 1D nanostructures [13–17]. Due to the geometry anisotropy, the emission intensity of $I_{∥}$ can be very different from that of $I_{⊥}$ , where $I_{∥}$ and $I_{⊥}$ are the emission intensities with the polarization parallel and perpendicular to the long axis of 1D nanostructures, respectively. To investigate the anisotropy of the emission in the oblique-aligned ZnO NWAs, we performed the polarized PL measurement at room temperature. The polarized PL spectra of the NWAs with the electric field of the emission parallel ( $E_{PL} ∥ c$ ) and perpendicular ( $E_{PL} ⊥ c$ ) to the azimuthal direction of the long axis of oblique-aligned NWAs exhibit the near bandedge emission (NBE) at ~377 nm and the deep level emission (DLE) at ~550 nm, as shown in Fig. 6 [55]. Regardless of the polarization of the excitation, the PL has maximum polarized emission in the direction parallel to the azimuthal direction of the long axis of the NWAs. Therefore, we only show the polarized spectra with the polarization of the excitation perpendicular to the azimuthal direction of the long axis of oblique-aligned NWAs. In addition, the strong NBE is attributed to the high crystal quality of the NWAs examined by the HRTEM image in Fig. 1(c). For quantitative analysis, the observed polarization anisotropy is typically defined in terms of the polarization ratio (also known as the degree of polarization) [15]

Fig. 6 PL spectra of the oblique-aligned ZnO NWAs at different polarization directions in (a) the NBE and (b) the DLE regions.

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ρ = \frac{I_{∥} - I_{⊥}}{I_{∥} + I_{⊥}}

Accordingly, the polarization ratios of NBE (ρ = 0.10) and DLE (ρ = 0.43) are obtained. The origin of polarization can be explained by the confinement of optical electric field, resulting in the anisotropic electric field distribution of the emission due to the large dielectric contrast between the NWs and their surroundings [19,56–59]. In dielectric contrast mechanism, the magnitude of an emitted electromagnetic wave with the electric field parallel to the NW long axis is higher than that perpendicular to the NW long axis. The high degree of polarization occurs in the small ratio of the NW diameter to the emission wavelength in air [59]. Consequently, the polarization ratio of DLE (ρ = 0.43) is higher than that of NBE (ρ = 0.10) in the oblique-aligned ZnO NWAs. Due to the existence of strong dielectric contrast effects, the optical anisotropy of NWA layers could be altered by simply varying the dielectric constant of the surrounding medium, oblique-angle for sputtering, or filling factor of ZnO, which is under investigation. One should note that the emission polarization ratio of our oblique-aligned NWA layers is lower than that of the single NW in previous studies [15,17]. This is because ZnO NWAs are very close to each other whereas a freestanding NW surrounded by air exhibits large dielectric contrast between the NW and its surroundings [15,17]. Furthermore, polarization anisotropy of NWs has been explained by quantum confinement-induced valence band mixing in some previous studies as well [60,61]. However, oblique-aligned ZnO NWAs with diameters of 80–100 nm much higher than the exciton Bohr radius (~2.34nm) are not applicable to the quantum confinement mechanism [62].

In summary, a robust, simple method of fabricating the single crystalline NWA layer with oblique angles ranging from 75° to 85° as an optically anisotropic material using the modified OAD and hydrothermal growth has been demonstrated. The oblique-aligned ZnO NWAs have giant in-plane birefringence ( $Δ n ≅$ 0.11). The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only can be applied to passive photonic components but also open up the possibility of important technological applications in polarized light sensing and emission devices.

Acknowledgment

The authors thank Prof. Shangjr Gwo and Dr. Hung-Ying Chen for fruitful discussion on the anisotropic emission. The work was supported by the National Science Council Grant No. 99- 2112-M-002-024-MY3, 100–2917-I-002–009, 98-2623-E-002-004-ET, and 100-2218-E-008-015.

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Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

Abstract

Acknowledgment

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Figures (6)

Equations (3)

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