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Gallium-nitride nanorods serve as subwavelength optical media

Vertically self-aligned, GaN nanorod arrays grown by plasma-assisted molecular-beam epitaxy demonstrate properties with important implications for optoelectronic applications.
17 February 2009, SPIE Newsroom. DOI: 10.1117/2.1200902.1524

GaN-based LEDs and laser diodes have become the devices of choice for optoelectronic applications operating in the short-wavelength range. One of the remaining challenges for improving GaN-based LEDs is to reduce their optical losses, which are caused by the high refractive index of GaN (~2.5).1 To overcome this difficulty, the low optical reflection from low-effective-refractive-index (low-n) nanorod arrays could be exploited.

To date, there have been few reports about light-extraction enhancement using transparent, conductive indium-tin oxide2 and zinc oxide3 nanorod arrays deposited on top of GaN LEDs. But GaN-nanorod arrays have been shown to have the same physical and thermal properties as the bulk GaN crystal, making them suitable and durable for high-power, GaN-based optoelectronic applications. By analyzing the reflectivity interference fringes, we have quantitatively determined that GaN-nanorod arrays behave as low-n transparent media in the entire visible spectra region.4 Moreover, the polarized properties of single GaN nanorods have been demonstrated and studied in detail.5 By measuring linearly polarized photoluminescence (PL) from individual, isolated GaN nanorods with diameters in the subwavelength regime (30–90nm), we are able to present clear experimental evidence for the size dependence of polarization anisotropy. This can resolve the long-standing issue related to the giant luminescence-polarization anisotropy observed from various semiconductor nanorods and nanowires.

In our investigations, high-density, vertically aligned (along the wurtzite c-axis) GaN nanorod arrays were grown using a catalyst-free, self-organized method based on plasma-assisted molecular-beam epitaxy (PA-MBE).6Judging from results of field-emission, scanning-electron microscopy (FE-SEM), both the average rod diameter and nearest-neighbor separation are in the subwavelength regime of visible light. There are two major advantages of using PA-MBE-grown GaN-nanorod arrays. First, the thickness and filling ratio (volume fraction) of the GaN-nanorod array can be tuned by controlling the growth parameters. And, the interfaces formed by PA-MBE are very abrupt, allowing for interferometer-type measurements.

We observed strong modulations for all nanorod arrays in the reflection spectra measured in the transparent-spectral region. This can be attributed to the effects of Fabry-Pérot microcavities formed within the nanorod arrays by optically flat air/nanorod and nanorod/substrate interfaces. Figure 1 shows a representative normal-incident reflectivity spectrum in the near-ultraviolet and visible regions of a GaN-nanorod array. Flat and featureless reflectivity spectra can be observed in the opaque region. Strongly modulated reflectivity spectra are measured in the transparent region. Using an analysis technique based on the spectral positions of the reflectivity fringes, we determined the effective refractive index of the GaN-nanorod array (neff): see Figure 1. The important finding in this analysis is that the effective refractive indices of nanorod arrays in the transparent region can be approximated to the entire dispersion relationship of bulk GaN reflective index by multiplying with a constant of ~0.65. This value also agrees with that obtained for the opaque region using a filling ratio of ~0.5.

Figure 1. A vertically aligned GaN nanorod array behaves as a low-refractive-index optical medium in both the transparent and opaque spectral regions. The effective refractive index in this medium can be directly related to the filling ratio of GaN nanorods in the GaN/air composite. (neff): effective refractive index. R(%): reflectivity in percentage.

Several semiconductor nanowires have giant emission anisotropy, which is highly desirable in a variety of applications such as polarized LEDs and flat-panel displays. J. Wang and colleagues proposed that the phenomenon is caused by the large dielectric contrast between the nanowire and the surrounding environment, as opposed to quantum-confinement effects.7 H. E. Ruda and A. Shik calculated the detailed effects of dielectric confinement for semiconductor nanowires.8 One important conclusion of their study is the prediction of strong size-dependence of emission anisotropy in the subwavelength regime. Therefore, it is critical to study individual, isolated nanorods with a size distribution within that regime. For our work, the GaN nanorod samples for the polarized PL measurements were prepared with a nanomanipulator system installed inside an FE-SEM. Figure 2 shows the linearly polarized PL signal from a single GaN nanorod. The inset shows the FE-SEM image of the measured GaN nanorod. The intensity of polarized PL with EPL||c (light polarization electric field component E parallel to the c-axis) is much larger than that with EPLc (light polarization electric field component E perpendicular to the c-axis). The polarization ratio can reach as high as 0.9, which is close to highest record.7 Our study confirmed a strong dependence of polarization ratio on the rod diameter. For GaN nanorods in this subwavelength regime, the effects of optical confinement are mainly responsible for the luminescence-polarization anisotropy we observed.

Figure 2. Linearly polarized photoluminescence (PL) from an isolated GaN nanorod (length: 1.2 microns, diameter: 40nm). By measuring linearly polarized PL from individual, isolated GaN nanorods with the rod diameters in the subwavelength regime (30–90nm), there is clear experimental evidence for the size dependence of polarization anisotropy consistent with the theoretical model based on subwavelength optical confinement in GaN nanorods. The inset shows the FE-SEM image of the measured GaN nanorod. The intensity of polarized PL with EPL||c(light polarization electric field component E parallel to the c-axis) is much larger than that with EPLc(light polarization electric field component E perpendicular to the c-axis). a.u.: arbitrary units.

In summary, we have studied the subwavelength optical properties of GaN nanorods in detail. The properties we have reported for GaN nanorods also can be found for other semiconductor materials and optical measurements. Because of the superior material properties of GaN nanorods, such as optical transparency, availability of n- and p-type conductivity, and excellent thermal and chemical stabilities, our results could have important implications for nanophotonics and optoelectronics applications.

We would like to thank the National Science Council, Taiwan, for financial support. The reflectivity measurements of GaN-nanorod arrays were performed in the Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan.

Shangjr Gwo, Hung-Ying Chen 
Department of Physics
National Tsing-Hua University, Taiwan
Hsinchu, Taiwan

Shangjr Gwo is a professor of physics at National Tsing-Hua University. His research interests include growth, fundamental studies, and applications of III-nitride nanostructures and heterostructures.

Hung-Ying Chen is a PhD candidate in physics at National Tsing-Hua University. His dissertation work is focused on optical properties of group-III nitride nanorods grown by plasma-assisted, molecular-beam epitaxy.