OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 17 — Aug. 18, 2008
  • pp: 13465–13475
« Show journal navigation

Polarized photoluminescence from single GaN nanorods: Effects of optical confinement

Hung-Ying Chen, Yu-Chen Yang, Hon-Way Lin, Shih-Cheng Chang, and Shangjr Gwo  »View Author Affiliations


Optics Express, Vol. 16, Issue 17, pp. 13465-13475 (2008)
http://dx.doi.org/10.1364/OE.16.013465


View Full Text Article

Acrobat PDF (3857 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

By measuring linearly polarized photoluminescence (PL) from single, isolated gallium nitride (GaN) nanorods with the rod diameters in the subwavelength regime (30–90 nm), we present clear evidence for size dependence of polarization anisotropy. The maximum polarization ratio at room temperature (~0.9 with emission and excitation light polarized parallel to the long axis of nanorod) occurs at the rod diameter of ~40 nm. The experimental data are compared with the recent theoretical model proposed for thick semiconductor nanowires. It is concluded that the optical confinement effects in this size regime play an important role in the observed giant polarization anisotropy. Furthermore, we have performed a temperature-dependent study of polarized PL to show the importance of internal emission anisotropy at low temperatures.

© 2008 Optical Society of America

1. Introduction

Optical properties of one-dimensional (1-D) nanostructures (nanowires, nanorods, nanopillars, and nanotubes, etc.) have been intensively studied during the last decade. One of the unique properties in the 1-D nanostructures is the strong polarization anisotropy of emission/absorption spectra. Due to the material and/or geometry anisotropy, the emission/absorption intensity of I , in which the emission/absorption light is polarized parallel to the long axis of 1-D nanostructures, can be very different from that of I , in which the emission/absorption light is polarized perpendicular to it. The observed polarization anisotropy is typically defined in terms of the polarization ratio (also called as the degree of polarization)

ρ=III+I.

In the early studies, 1-D nanostructures were fabricated primarily by lithography [1–3

1. M. Kohl, D. Heitmann, P. Grambow, and K. Ploog, “One-dimensional magneto-excitons in GaAs/AlxGa1-xAs quantum wires,” Phys. Rev. Lett. 63, 2124–2127 (1989). [CrossRef] [PubMed]

] and epitaxy [4

4. T. Someya, H. Akiyama, and H. Sakaki, “Laterally squeezed excitonic wave function in quantum wires,” Phys. Rev. Lett. 74, 3664–3667 (1995). [CrossRef] [PubMed]

,5

5. H. Akiyama, T. Someya, and H. Sakaki, “Optical anisotropy in 5-nm-scale T-shaped quantum wires fabricated by the cleaved-edge overgrowth method,” Phys. Rev. B 53, R4229–R4232 (1996). [CrossRef]

] and the III-arsenide (AlAs, GaAs, InAs, and their alloys) quantum wire structures were often used. Ils et al. suggested that the polarization ratio is a function of the lateral width [3

3. P. Ils, Ch. Gréus, A. Forchel, V. D. Kulakovskii, N. A. Gippius, and S. G. Tikhodeev, “Linear polarization of photoluminescence emission and absorption in quantum-well wire structures: Experiment and theory,” Phys. Rev. B 51, 4272–4277 (1995). [CrossRef]

]. It was found that the polarization ratio is proportional to 1/Lx (Lx is the lateral width of nanowires) for thick wires and it saturates in the case of thin wires at a level of 0.2–0.6 depending on the structure of quantum wire. Later on, a giant anisotropy (~0.9) in the band gap emission with the dominant polarization parallel to the wire long axis was reported for single InP nanowires prepared by a bottom-up method [6

6. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and potodetection from single indium phosphide nanowires,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]

]. After that, many different kinds of nanowires were studied and similar phenomena were also reported [7–10

7. J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, “Optical cavity effects in ZnO nanowire lasers and waveguides,” J. Phys. Chem. B , 107, 8816–8828 (2003). [CrossRef]

]. For example, the polarization dependence on the emission wavelength has been found for the case of single ZnO nanowires [7

7. J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, “Optical cavity effects in ZnO nanowire lasers and waveguides,” J. Phys. Chem. B , 107, 8816–8828 (2003). [CrossRef]

]. Especially, the band gap emission of ZnO nanowires shows a negative polarization ratio (-0.65). This property has also been observed in CdS nanorods [9

9. D. Kulik, H. Htoon, C. K. Shih, and Y. Li, “Photoluminescence properties of single CdS nanorods,” J. Appl. Phys. 95, 1056–1063 (2004). [CrossRef]

]. Furthermore, polarized photoluminescence (PL) from individual CdSe nanowires grown parallel and perpendicular to the c axis of the wurtzite structure was reported [10

10. C. X. Shan, Z. Liu, and S. K. Hark, “Photoluminescence polarization in individual CdSe nanowires,” Phys. Rev. B 74, 153402 (2006). [CrossRef]

]. Shan et al. concluded that the polarization anisotropy of emission from CdSe nanowires does not depend on the growth direction, but is mainly determined by the elongated shape (aspect ratio) of nanowires.

Although the present experimental results are not completely consistent, it is generally acknowledged that there are two major mechanisms responsible for these phenomena. The first kind of mechanism attributes these phenomena to the quantum size effects [11–16

11. P. C. Sercel and K. J. Vahala, “Analytical technique for determining the polarization dependence of optical matrix elements in quantum wires with band-coupling effects,” Appl. Phys. Lett. 57, 545–547 (1990). [CrossRef]

]. In this mechanism, the quantum confinement of carriers in 1-D nanostructures causes the quantization of energy spectrum and the changes in optical transition matrix. It is important to note that, in order to be applicable to this mechanism, the size of nanostructures should be smaller than the exciton Bohr radius. In contrast, the second kind of mechanism is based on the confinement of optical electric field [17

17. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in semiconducting and metallic nanowires,” Phys. Rev. B 72, 115308 (2005). [CrossRef]

,18

18. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

] due to the variation in the dielectric constants of nanowires/nanorod (larger) and its environment (smaller), which can result in the anisotropic electric field distribution. This effect becomes dominant in thick semiconductor 1-D nanostructures and a strong size dependent behavior was predicted by Ruda et al. in the subwavelength regime, in which the rod diameter is smaller than the emission wavelength but much larger than the exciton Bohr radius [18

18. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

].

Comparing with other semiconductor nanowires, reports on optical properties of single GaN nanowires or nanorods are relatively scarce. Recently, a polarization-resolved PL study of single wurtzite GaN nanowires has been reported by Schlager et al. [19

19. J. B. Schlager, N. A. Sanford, K. A. Bertness, J. M. Barker, A. Roshko, and P. T. Blanchard, “Polarizationresolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy,” Appl. Phys. Lett. 88, 213106 (2006). [CrossRef]

]. Their experimental data showed that the observed excitonic transitions at low and room temperatures are in agreement with the optical selection rules of wurtzite crystal symmetry. However, the degree of PL polarization and its dependence on the sizes of GaN nanowires or nanorods have not been investigated in detail. In this work, by measuring PL from single, isolated GaN nanorods in the subwavelength regime (30–90 nm), we present clear evidence for strong size (rod diameter) dependence. Based on these results, it can be concluded that the optical confinement effects play an important role in the observed giant polarization anisotropy. Furthermore, we have performed a temperature-dependent study of PL polarization anisotropy to show the importance of internal emission anisotropy at low temperatures.

2. Sample preparation and optical measurements

Vertically aligned, wurtzite GaN nanorod arrays were grown on 3-inch Si(111) wafers by plasma-assisted molecular-beam epitaxy (PA-MBE). The details of growth process can be found elsewhere [20

20. H.-Y. Chen, H.-W. Lin, C.-H. Shen, and S. Gwo, “Structure and photoluminescence properties of epitaxially oriented GaN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy,” Appl. Phys. Lett. 89, 243105 (2006). [CrossRef]

]. The grown GaN nanorod arrays have been confirmed to be single crystalline and epitaxially oriented with the epitaxial relationship of <2 1̄ 1̄ 0>GaN‖ <1̄ 1 0>Si and <1 1̄ 00>GaN‖ <1 1 2̄>Si. Moreover, the GaN nanorods are nitrogen-polar with the axial direction pointed to the –c-axis ([000 1̄] direction). The samples of single, isolated GaN nanorods for the polarized PL measurements were prepared by using a four-probe nanomanipulator (Zyvex) installed inside a field-emission scanning electron microscope (FE-SEM, Ultra 55, Zeiss). A movie showing the nanorod manipulation process is shown online (Media 1 [3.6 MB]). Figure 1(a) shows the FE-SEM image taken at the region of a 2-µm-long GaN nanorod-array sample near the cleavage edge and with a tungsten probe positioned nearby. In such a region, numerous single nanorods and bundled nanorods can be found, and we can utilize sharp tungsten probes to manipulate single GaN nanorods to designated positions and orientations. During the manipulation process, when the distance between the tungsten probe and the GaN nanorod is small enough, single nanorods can be attracted to the commonly grounded tungsten probe by using the electron-beam-induced electrostatic force. As show in Fig. 1(b), one single GaN nanorod is attached to the probe by using this mechanism and is then removed far from the Si substrate. Afterwards, we can separate single GaN nanorods from the probe by using the van der Waals force when the contact area between the nanorod and the substrate is formed by moving the nanorod to the designated areas on a surface registered with a gold alignment pattern. Single nanorods can be separated from the probe because that the van der Waals force (with the substrate) is larger than electrostatic force (with the probe). Figure 1(c) shows four single GaN nanorods manipulated in this way onto a gold-patterned Si substrate. The inset in Fig. 1(c) shows the GaN nanorod with uniform diameter of 35 nm and length of 2 µm. Utilizing this technique, we can manipulate single nanorods according to specific requirements. For the polarized PL measurements, we selected GaN nanorods ranging in diameter of 30–90 nm and in length of 1.2–2 µm. The shorter nanorods were produced during the cleavage process of nanorod-array-covered Si substrates.

For the PL analysis, a home-built micro-PL (µ-PL) system was used in this study. Samples were excited with a continuous-wave HeCd laser operating at 325 nm, which was focused with a UV compatible object lens (Mitutoyo, NA=0.5) to a spot diameter of about 1 µm. The PL signal was collected with the same object lens and dispersed by a spectrometer (HR460, Jobin-Yvon) equipped with a liquid-nitrogen-cooled CCD detector. In the optical images, single GaN nanorods appeared to be small shadows. With reference to the FE-SEM images, we can position the focused laser spot to excite individually designated GaN nanorods by the relative positions between gold alignment pattern and GaN nanorods. The polarized PL data were carefully calibrated in order to eliminate the polarization dependence of the measurement setup. Using this procedure, the detected PL of c-plane GaN epilayers at similar emission energy has been confirmed to be unpolarized, as theoretically expected.

Fig. 1. FE-SEM images of nanorod manipulation process. (a) Image near the cleavage edge of a Si(111) substrate covered by a vertically aligned array of 2-µm-long GaN nanorods. A tungsten probe is positioned nearby. (b) A tungsten probe attached with a single GaN nanorod by electrostatic interaction. (c) Using this technique, four single, isolated GaN nanorods were positioned on a gold-patterned Si substrate with designated locations and orientations; the inset shows a GaN nanorod with uniform diameter of 35 nm and length of 2 µm. A movie showing the nanorod manipulation process is shown online (Media 1).

3. Results and discussion

In our µ-PL setup, the polarized PL measurements at room temperature were performed in the geometry that the incident direction (wave vector) of laser light is perpendicular to the long axis of GaN nanorod (k excc). Unlike the common k excc geometry for thin-film studies, we can distinguish two PL polarizations in the plane where the GaN nanorods reside: the σ polarization corresponds to the electric field perpendicular to the long axis (c-axis); and the π polarization corresponds to the electric field parallel to the c-axis. The emission properties of bulk GaN in the k excc geometry have been reported by Paskov et al. [21

21. P. P. Paskov, T. Paskova, P. O. Holtz, and B. Monemar, “Polarized photoluminescence study of free and bound excitons in free-standing GaN,” Phys. Rev. B 70, 035210 (2004). [CrossRef]

] using the cleaved sample edge of bulk GaN. Here, we investigate the emission properties of single, strain-free GaN nanorods in the k excc geometry. Figure 2(a) shows the PL dependence on the excitation polarization. The inset shows the FE-SEM image of the measured GaN nanorod (length: 1.2 µm, diameter: 40 nm). This rod is labeled as #R1 according to the nearest gold alignment pattern mark. As shown in Fig. 2(a), both of the PL spectra exhibit band gap emission (~3.4 eV) at room temperature and a higher PL intensity were obtained in the configuration that E excc (the electric field of excitation is parallel to the c-axis of GaN nanorod). This difference is mainly due to stronger optical absorption in the configuration that E excc. Similar results have also been reported on other semiconductor nanowires, such as InP [6

6. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and potodetection from single indium phosphide nanowires,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]

] and CdSe [10

10. C. X. Shan, Z. Liu, and S. K. Hark, “Photoluminescence polarization in individual CdSe nanowires,” Phys. Rev. B 74, 153402 (2006). [CrossRef]

].

Fig. 2. Photoluminescence spectra of single GaN nanorods at room temperature measured with the k excc geometry. (a) Dependence on the optical absorption polarization. The inset shows the FE-SEM image of the measured GaN (#R1) nanorod with a diameter of 40 nm. (b) and (c) show the polarized PL spectra with the electric field of optical excitation parallel and perpendicular to the c-axis, respectively. The measured polarization ratios are 0.9 and 0.84, respectively. (d) Polarization ratio measured from another GaN nanorod (#R3) using the same experimental configuration. The diameter of #R3 GaN nanorod is 80 nm, as shown in the inset.

The linearly polarized PL signal was measured by using a polarizer positioned in the luminescence collection pathway. In Figs. 2(b) and 2(c), we show the polarized PL of #R1 GaN nanorod with the electric field of luminescence parallel (E PL ‖ c) and perpendicular (E PLc) to the long axis (c-axis) of GaN nanorods. In Fig. 2(b), the excitation configuration is E exc ‖ c, and the intensity of polarized PL with E PL ‖ c is much larger than that with E PLc. The polarization ratio can reach as high as 0.9. This value is close to highest record that observed in individual InP nanowires [6

6. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and potodetection from single indium phosphide nanowires,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]

]. Figure 2(c) shows the polarized PL using the other excitation configuration such that E excc and the polarization ratio can also reach about 0.84. Although the polarization ratios with different excitation configurations are not in full agreement, we believe that the polarization anisotropy does not depend on the polarization of excitation. And, the slight difference is mainly due to the misalignment between the axes of the polarizer and the nanorod during different measurements. Figure 2(d) shows the polarized PL from a different single GaN nanorod (labeled as #R3) using the same measurement configuration as that shown in Fig. 2(b). The #R3 GaN nanorod has a length of 2 µm and a diameter of 80 nm. And, the measured degree of linear polarization is only 0.16. This value is quite small compared with that of #R1 GaN nanorod. Therefore, in order to understand the origin of large difference in polarization anisotropy from different GaN nanorods, it is necessary to study a large number of singles GaN nanorods with different diameters. The nanorod manipulation technique adopted here is therefore very handy for these measurements.

As mention in introduction, there are two main mechanisms proposed for interpreting the giant linear polarization, which include the quantum size effects and the optical confinement effects. Both of these two mechanisms depend on the sizes of nanostructures [11–18

11. P. C. Sercel and K. J. Vahala, “Analytical technique for determining the polarization dependence of optical matrix elements in quantum wires with band-coupling effects,” Appl. Phys. Lett. 57, 545–547 (1990). [CrossRef]

]. However, for the case of carrier confinement, the size dependence not only determines the degree of polarization but also affects the emission energy [11–16

11. P. C. Sercel and K. J. Vahala, “Analytical technique for determining the polarization dependence of optical matrix elements in quantum wires with band-coupling effects,” Appl. Phys. Lett. 57, 545–547 (1990). [CrossRef]

]. Thus, the experiments for size-dependence of polarized PL should be carried out also with an analysis on the emission energy. For the size-dependent measurements, we selected and measured a number of GaN nanorods, which have diameters in the range of 30–90 nm and lengths in the range of 1.2–2 µm. Figure 3(a) shows the relation between the diameters of selected nanorods and their PL peak energies at room temperature. Clearly, there is no significant indication that the peak energy of PL depends on the diameter of nanorods. Thus, the quantum confinement effect of carrier should be small and it can be safely neglected for these samples. This result is in agreement with the fact that the size of GaN nanorods in this work (30–90 nm) is much larger than the exciton Bohr radius in GaN (<10 nm).

Fig. 3. Size dependence of polarized photoluminescence from single GaN nanorods (k excc). (a) The peak energies of PL from single GaN nanorods with different rod diameters. The dashed lines show the averaged values of peak energy with two PL polarizations. (b) The polarization ratio of PL from single GaN nanorods with different diameters using the measurement configuration that the electric field of excitation is parallel to the c-axis. The dashed line is the calculated result considering dielectric confinement of optical electric field with ε=6 and isotropic internal emissions (see text for details).

In contrast, the strong size dependence can be explained well using the optical confinement effects [17

17. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in semiconducting and metallic nanowires,” Phys. Rev. B 72, 115308 (2005). [CrossRef]

,18

18. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

]. In this approach, the internal field E i is described by the Helmholtz equation. Using cylindrical coordinates (r,ϕ,z), inside the nanorod (ra, where a is radius of nanorod) E i can be expressed as

1rr(rEir)+1r22Eiϕ2+2Eiz2+εω2c2Ei=0,
(1)

where ω is the light frequency and c is the light velocity. Outside the nanowires (r>a), the equations should be modified by replacing ε with ε 0. The solution of this equation depends on the relative orientation of external electric field E e(r,ϕ,z) and the nanowire long axis [18

18. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

,23

23. V. V. Batygin and I. N. Toptygin, Problems in Electrodynamics (Academic, 1978).

]. Also, the factor of 2ε 0/(ε+ε 0) can be derived using the limit of ωa/c→0. In general, the luminescence properties can be obtained by using the general formulas for a field created by a dipole with arbitrary position r 0 followed by integration over r 0, which can only be performed numerically [24

24. W. C. Chew, Waves and Fields in Inhomogeneous Media (Van Nostrand Reinhold, 1990).

]. Instead of this method, Ruda and Shik modeled their case simply by an effective emitting dipole d(r,ϕ,z) at the axis (r=z=0).

According to the model proposed by Ruda et al. [18

18. H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

], for original dipole moment oriented along the long axis (d 0z), the effective dipole moment can be written as dz=d 0z Az, where Az is a function of dielectric constant (ε), light frequency (ω), and radius of nanorods (a). Thus, Az can be considered as an effective parameter that comes from the material properties and size. Similarly, for the perpendicular dipole (d 0x), the effective dipole moment can be written as dx=d 0x Ax. Therefore, for cylindrical nanowire dx=dy, and the intensity ratio for different light polarization is given by

I//I=2dz2+dx23dx2,
(2)

where I and I is the luminescence intensity with electric field parallel and perpendicular to the long axis, respectively. And, the polarization ratio can be given by

ρ=I//II//+I=dz2dx2dz2+2dx2.
(3)

For isotropic internal emission (d 0z=d 0x), the polarization ratio can be described completely by effective parameters

ρ=Az2Ax2Az2+2Ax2ρ0.
(4)

Here, ρ 0 depends only on the properties of dielectric media, which are dielectric constant (ε) and the ratio of the nanorod diameter to the emission wavelength in vacuum (a/λ 0). Figure 4(a) shows the numerical results of ρ 0 with varying dielectric constants under the conditions of isotropic internal emission. In short, the high degree of polarization occurs prominently in thin nanorods (diameters in the subwavelength regime) with a high dielectric constant. And, independent of the dielectric constant, the polarization anisotropy would not be obvious when the nanorod diameter is much larger than the emission wavelength (a/λ 0ωa/c≫1).

For anisotropic internal emission, we should go back to Eq. (3). Here, we define a parameter βd 2 0x/d 2 0z to quantify the anisotropy of internal emission. Thus, for the case of β=1, linear polarization can be described completely by ρ 0 as discussed above. Figure 4(b) shows the simulated polarization ratio with different values of β. Apparently, higher degree of polarization can be obtained under the situation of small β. Moreover, the value of polarization ratio converges to -0.5 with increasing β, which is in accordance with Eq. (3) at the limit of β≫1. Furthermore, in the case of ωa/c→∞, which can be considered as the case of bulk crystal, optical confinement can be ignored and the degree of polarization is mainly attributed to the anisotropy of internal emission.

Fig. 4. Numerical results for simulating the effects of optical (dielectric) confinement. (a) Results obtained by varying dielectric constants; (b) Results obtained by varying ratios of internal dipole moment along two orthogonal directions.

Fig. 5. Numerical results of polarization ratio using the dielectric confinement model. (a) Results obtained by varying dielectric constants for GaN nanorods; (b) Result obtained by using (β,ε)=(1.5,6.5).

Fig. 6. (a) FE-SEM images of four different GaN bundles, among which the #V5 GaN bundle stands vertically on the Si substrate. (b) Photoluminescence spectra of GaN bundles at low temperature (~10 K). For clarity, the inset shows the PL spectra in semi-logarithm scale.

Figure 7 shows the polarized PL measurement for the #R1 GaN nanorod at ~20 K, the strong near-band-gap emission is accompanied with a shoulder peak occurring at around 360 nm in the E PLc polarization. Interestingly, the intensity of this shoulder can be varied by moving the focused laser spot with respect to the initial growth end of #R1 GaN nanorod, which corresponds to the interfaced region with the Si substrate during PA-MBE growth. Thus, the shoulder peak should correspond to the defect emission (Y 2) at the GaN/Si interface. From the normalized PL plot (inset), we can identify that the main exciton emissions correspond to D 0 XA (3.472 eV) and D 0 XB (3.477 eV), which are in good agreement with the selection rules of optically active exciton emissions in strain-free wurtzite GaN [19

19. J. B. Schlager, N. A. Sanford, K. A. Bertness, J. M. Barker, A. Roshko, and P. T. Blanchard, “Polarizationresolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy,” Appl. Phys. Lett. 88, 213106 (2006). [CrossRef]

,21

21. P. P. Paskov, T. Paskova, P. O. Holtz, and B. Monemar, “Polarized photoluminescence study of free and bound excitons in free-standing GaN,” Phys. Rev. B 70, 035210 (2004). [CrossRef]

].

Fig. 7. Measurement of polarized photoluminescence from the #R1 GaN nanorod at low temperature (~20 K). The inset shows the corresponding PL peak positions in the plot of normalized intensity. Comparing with the room-temperature measurement on the same nanorod, the polarization ratio at low temperature is completely quenched due to the much enhanced anisotropy of internal emissions due to the neutral-donor-bound excitons.

4. Conclusions

In summary, size- and temperature-dependent polarized PL from single, isolated GaN nanorods have been measured. The excitonic emissions of GaN nanorod have been confirmed to follow the optical selection rules in wurtzite GaN crystal. It is concluded that the optical confinement effects dominate the linearly polarized properties of GaN nanorods with diameters in the subwavelength regime (30–90 nm). At room temperature, the size-dependence of polarization ratio can be described well using the recently proposed theoretical model based on optical (dielectric) confinement. At low temperatures, the degree of polarization could be dramatically modified by the anisotropy of internal emissions originating from the neutral-donor-bound excitons. Due to the their general applicability, we propose that the polarized luminescence properties reported here for GaN can also be found for other semiconductor materials and for other optical measurements, such as electroluminescence and cathodoluminescence.

Acknowledgments

This work was supported in part by the National Nanoscience and Nanotechnology Project (NSC 96-2120-M-007-008) and the “Foresight Taiwan” Program (NSC 96-3011-P-007-004).

References and links

1.

M. Kohl, D. Heitmann, P. Grambow, and K. Ploog, “One-dimensional magneto-excitons in GaAs/AlxGa1-xAs quantum wires,” Phys. Rev. Lett. 63, 2124–2127 (1989). [CrossRef] [PubMed]

2.

U. Bockelmann and G. Bastard, “Interband absorption in quantum wires. I. Zero-magnetic-field case,” Phys. Rev. B 45, 1688–1699 (1992). [CrossRef]

3.

P. Ils, Ch. Gréus, A. Forchel, V. D. Kulakovskii, N. A. Gippius, and S. G. Tikhodeev, “Linear polarization of photoluminescence emission and absorption in quantum-well wire structures: Experiment and theory,” Phys. Rev. B 51, 4272–4277 (1995). [CrossRef]

4.

T. Someya, H. Akiyama, and H. Sakaki, “Laterally squeezed excitonic wave function in quantum wires,” Phys. Rev. Lett. 74, 3664–3667 (1995). [CrossRef] [PubMed]

5.

H. Akiyama, T. Someya, and H. Sakaki, “Optical anisotropy in 5-nm-scale T-shaped quantum wires fabricated by the cleaved-edge overgrowth method,” Phys. Rev. B 53, R4229–R4232 (1996). [CrossRef]

6.

J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, “Highly polarized photoluminescence and potodetection from single indium phosphide nanowires,” Science 293, 1455–1457 (2001). [CrossRef] [PubMed]

7.

J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, “Optical cavity effects in ZnO nanowire lasers and waveguides,” J. Phys. Chem. B , 107, 8816–8828 (2003). [CrossRef]

8.

J. Qi, A. M. Belcher, and J. M. White, “Spectroscopy of individual silicon nanowires,” Appl. Phys. Lett. 82, 2616–2618 (2003). [CrossRef]

9.

D. Kulik, H. Htoon, C. K. Shih, and Y. Li, “Photoluminescence properties of single CdS nanorods,” J. Appl. Phys. 95, 1056–1063 (2004). [CrossRef]

10.

C. X. Shan, Z. Liu, and S. K. Hark, “Photoluminescence polarization in individual CdSe nanowires,” Phys. Rev. B 74, 153402 (2006). [CrossRef]

11.

P. C. Sercel and K. J. Vahala, “Analytical technique for determining the polarization dependence of optical matrix elements in quantum wires with band-coupling effects,” Appl. Phys. Lett. 57, 545–547 (1990). [CrossRef]

12.

P. C. Sercel and K. J. Vahala, “Polarization dependence of optical absorption and emission in quantum wires,” Phys. Rev. B 44, 5681–5691 (1991). [CrossRef]

13.

C. R. McIntyre and L. J. Sham, “Theory of luminescence polarization anisotropy in quantum wires,” Phys. Rev. B 45, 9443–9446 (1992). [CrossRef]

14.

M. P. Persson and H. Q. Xu, “Giant polarization anisotropy in optical transitions of free-standing InP nanowires,” Phys. Rev. B 70, 161310(R) (2004). [CrossRef]

15.

M. Califano and A. Zunger, “Anisotropy of interband transitions in InAs quantum wires: An atomistic theory,” Phys. Rev. B 70, 165317 (2004). [CrossRef]

16.

A. V. Maslov and C. Z. Ning, “Radius-dependent polarization anisotropy in semiconductor nanowires,” Phys. Rev. B 72, 161310(R) (2005). [CrossRef]

17.

H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in semiconducting and metallic nanowires,” Phys. Rev. B 72, 115308 (2005). [CrossRef]

18.

H. E. Ruda and A. Shik, “Polarization-sensitive optical phenomena in thick semiconducting nanowires,” J. Appl. Phys. 100, 024314 (2006). [CrossRef]

19.

J. B. Schlager, N. A. Sanford, K. A. Bertness, J. M. Barker, A. Roshko, and P. T. Blanchard, “Polarizationresolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy,” Appl. Phys. Lett. 88, 213106 (2006). [CrossRef]

20.

H.-Y. Chen, H.-W. Lin, C.-H. Shen, and S. Gwo, “Structure and photoluminescence properties of epitaxially oriented GaN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy,” Appl. Phys. Lett. 89, 243105 (2006). [CrossRef]

21.

P. P. Paskov, T. Paskova, P. O. Holtz, and B. Monemar, “Polarized photoluminescence study of free and bound excitons in free-standing GaN,” Phys. Rev. B 70, 035210 (2004). [CrossRef]

22.

L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, 2nd ed. (Elsevier Butterworth-Heinemann, 1984).

23.

V. V. Batygin and I. N. Toptygin, Problems in Electrodynamics (Academic, 1978).

24.

W. C. Chew, Waves and Fields in Inhomogeneous Media (Van Nostrand Reinhold, 1990).

25.

H.-Y. Chen, H.-W. Lin, C.-Y. Wu, W.-C. Chen, J.-S. Chen, and S. Gwo, “Gallium nitride nanorod arrays as low-refractive-index transparent media in the entire visible spectral region,” Opt. Express 16, 8106–8116 (2008). [CrossRef] [PubMed]

26.

B. Gil and O. Briot, “Internal structure and oscillator strengths of excitons in strained α-GaN,” Phys. Rev. B 55, 2530–2534 (1997). [CrossRef]

27.

M. A. Reshchikov and H. Morkoç “Luminescence properties of defects in GaN,” J. Appl. Phys. 97, 061301 (2005). [CrossRef]

28.

H. Kawanishi, E. Niikura, M. Yamamoto, and S. Takeda, “Experimental energy difference between heavy-or light-hole valence band and crystal-field split-off-hole valence band in AlxGa1-xN,” Appl. Phys. Lett. 89, 251107 (2006). [CrossRef]

29.

S. Nakagawa, H. Tsujimura, K. Okamoto, M. Kubota, and H. Ohta, “Temperature dependence of polarized electroluminescence from nonpolor m-plane InGaN-based light emitting diodes,” Appl. Phys. Lett. 91, 171110 (2007). [CrossRef]

OCIS Codes
(160.6000) Materials : Semiconductor materials
(250.5230) Optoelectronics : Photoluminescence
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(160.4236) Materials : Nanomaterials
(310.6628) Thin films : Subwavelength structures, nanostructures

ToC Category:
Materials

History
Original Manuscript: July 3, 2008
Revised Manuscript: August 11, 2008
Manuscript Accepted: August 12, 2008
Published: August 15, 2008

Citation
Hung-Ying Chen, Yu-Chen Yang, Hon-Way Lin, Shih-Cheng Chang, and Shangjr Gwo, "Polarized photoluminescence from single GaN nanorods: Effects of optical confinement," Opt. Express 16, 13465-13475 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-17-13465


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. M. Kohl, D. Heitmann, P. Grambow, and K. Ploog, "One-dimensional magneto-excitons in GaAs/AlxGa1-xAs quantum wires," Phys. Rev. Lett. 63, 2124-2127 (1989). [CrossRef] [PubMed]
  2. U. Bockelmann and G. Bastard, "Interband absorption in quantum wires. I. Zero-magnetic-field case," Phys. Rev. B 45, 1688-1699 (1992). [CrossRef]
  3. P. Ils, Ch. Gréus, A. Forchel, V. D. Kulakovskii, N. A. Gippius, and S. G. Tikhodeev, "Linear polarization of photoluminescence emission and absorption in quantum-well wire structures: Experiment and theory," Phys. Rev. B 51, 4272-4277 (1995). [CrossRef]
  4. T. Someya, H. Akiyama, and H. Sakaki, "Laterally squeezed excitonic wave function in quantum wires," Phys. Rev. Lett. 74, 3664-3667 (1995). [CrossRef] [PubMed]
  5. H. Akiyama, T. Someya, and H. Sakaki, "Optical anisotropy in 5-nm-scale T-shaped quantum wires fabricated by the cleaved-edge overgrowth method," Phys. Rev. B 53, R4229-R4232 (1996). [CrossRef]
  6. J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, "Highly polarized photoluminescence and potodetection from single indium phosphide nanowires," Science 293, 1455-1457 (2001). [CrossRef] [PubMed]
  7. J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, "Optical cavity effects in ZnO nanowire lasers and waveguides," J. Phys. Chem. B,  107, 8816-8828 (2003). [CrossRef]
  8. J. Qi, A. M. Belcher, and J. M. White, "Spectroscopy of individual silicon nanowires," Appl. Phys. Lett. 82, 2616-2618 (2003). [CrossRef]
  9. D. Kulik, H. Htoon, C. K. Shih, and Y. Li, "Photoluminescence properties of single CdS nanorods," J. Appl. Phys. 95, 1056-1063 (2004). [CrossRef]
  10. C. X. Shan, Z. Liu, and S. K. Hark, "Photoluminescence polarization in individual CdSe nanowires," Phys. Rev. B 74, 153402 (2006). [CrossRef]
  11. P. C. Sercel and K. J. Vahala, "Analytical technique for determining the polarization dependence of optical matrix elements in quantum wires with band-coupling effects," Appl. Phys. Lett. 57, 545-547 (1990). [CrossRef]
  12. P. C. Sercel and K. J. Vahala, "Polarization dependence of optical absorption and emission in quantum wires," Phys. Rev. B 44, 5681-5691 (1991). [CrossRef]
  13. C. R. McIntyre and L. J. Sham, "Theory of luminescence polarization anisotropy in quantum wires," Phys. Rev. B 45, 9443-9446 (1992). [CrossRef]
  14. M. P. Persson and H. Q. Xu, "Giant polarization anisotropy in optical transitions of free-standing InP nanowires," Phys. Rev. B 70, 161310(R) (2004). [CrossRef]
  15. M. Califano and A. Zunger, "Anisotropy of interband transitions in InAs quantum wires: An atomistic theory," Phys. Rev. B 70, 165317 (2004). [CrossRef]
  16. A. V. Maslov and C. Z. Ning, "Radius-dependent polarization anisotropy in semiconductor nanowires," Phys. Rev. B 72, 161310(R) (2005). [CrossRef]
  17. H. E. Ruda and A. Shik, "Polarization-sensitive optical phenomena in semiconducting and metallic nanowires," Phys. Rev. B 72, 115308 (2005). [CrossRef]
  18. H. E. Ruda and A. Shik, "Polarization-sensitive optical phenomena in thick semiconducting nanowires," J. Appl. Phys. 100, 024314 (2006). [CrossRef]
  19. J. B. Schlager, N. A. Sanford, K. A. Bertness, J. M. Barker, A. Roshko, and P. T. Blanchard, "Polarization-resolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy," Appl. Phys. Lett. 88, 213106 (2006). [CrossRef]
  20. H.-Y. Chen, H.-W. Lin, C.-H. Shen, and S. Gwo, "Structure and photoluminescence properties of epitaxially oriented GaN nanorods grown on Si(111) by plasma-assisted molecular-beam epitaxy," Appl. Phys. Lett. 89, 243105 (2006). [CrossRef]
  21. P. P. Paskov, T. Paskova, P. O. Holtz, and B. Monemar, "Polarized photoluminescence study of free and bound excitons in free-standing GaN," Phys. Rev. B 70, 035210 (2004). [CrossRef]
  22. L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, 2nd ed. (Elsevier Butterworth-Heinemann, 1984).
  23. V. V. Batygin and I. N. Toptygin, Problems in Electrodynamics (Academic, 1978).
  24. W. C. Chew, Waves and Fields in Inhomogeneous Media (Van Nostrand Reinhold, 1990).
  25. H.-Y. Chen, H.-W. Lin, C.-Y. Wu, W.-C. Chen, J.-S. Chen, and S. Gwo, "Gallium nitride nanorod arrays as low-refractive-index transparent media in the entire visible spectral region," Opt. Express 16, 8106-8116 (2008). [CrossRef] [PubMed]
  26. B. Gil and O. Briot, "Internal structure and oscillator strengths of excitons in strained α-GaN," Phys. Rev. B 55, 2530-2534 (1997). [CrossRef]
  27. M. A. Reshchikov and H. Morkoç "Luminescence properties of defects in GaN," J. Appl. Phys. 97, 061301 (2005). [CrossRef]
  28. H. Kawanishi, E. Niikura, M. Yamamoto, and S. Takeda, "Experimental energy difference between heavy- or light-hole valence band and crystal-field split-off-hole valence band in AlxGa1??xN," Appl. Phys. Lett. 89, 251107 (2006). [CrossRef]
  29. S. Nakagawa, H. Tsujimura, K. Okamoto, M. Kubota, and H. Ohta, "Temperature dependence of polarized electroluminescence from nonpolor m-plane InGaN-based light emitting diodes," Appl. Phys. Lett. 91, 171110 (2007). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Supplementary Material


» Media 1: MOV (3573 KB)     

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited