GIPO newsletter

This study analyzes the optical polarization characteristics and internal quantum efficiency of AlGaN based polar, nonpolar, and semipolar deep ultra-violet(UV) LED. A 1D model is used to solve drift-diffusion, Poisson equations and 6x6 k.p Schodinger equations to investigate band structure and emission characteristics. The light emission polarization ratios of nonpolar and semipolar AlGaN based deep UV LED with different Al compositions and injection current were studied. The studies shows that the optical polarization of c-plane AlGaN based deep UV LED is dominated by the out-plane polarization as the Al composition increases. And for the nonpolar structures, the light polarization direction is mainly dominated by in-plane polarized light which is good for surface emitting. Although the anisotropic polarization property of semipolar structures is not as strong as the nonpolar structure, it can be another choice if the growth speed is faster than the nonpolar plane. Finally, the study discusses IQE behavior through changing the p-type activation energy, quantum well layer number, and Schottky barrier height. The detail work can be found in our recent publications.

Surface Effect and Device Application of Nanostructured ZnO

Nanostructured ZnO has attracted intensive research efforts for its versatile applications such as nanogenerators, stain sensors, field-effect transistors, light emitting diodes, gas sensors, optical sensors, and resistive memory. The surface effects including surface band bending, chemisorption/photodesorption near surfaces, native surface defects/states, and surface roughness are more pronounced in the nanostructures than that in thin film and bulk counterparts due to the structural uniqueness and the ultrahigh surface-to-volume ratio of ZnO nanostructures. For the past years, we have continued to understand how the physical properties are affected by shrinking the dimension of ZnO for exceeding state-of-the-art planar devices and developed the novel application of ZnO nanostructures utilizing the surface effect.

Two infrared emission modes with different wavelengths and orthogonal polarization in a waveguide thermal emitter

We investigated an Au/SiO₂/Au waveguide thermal emitter incorporating a metallic grating embedded in the SiO₂layer (Fig. 1). The metal grating acts as a beam splitter, dividing the device into two waveguide structures determined by the polarization of the waveguide modes. The thermal radiation spectrum exhibits two peaks with orthogonal polarization (Fig. 2). The emitted wavelengths of the two waveguide modes can be adjusted by controlling the thickness of the SiO₂ layers on both sides of the metallic grating (Fig. 3(a) and 3(b)). The emission peaks have ratios of the full width at half maximum to the peak wavelength of 0.057 and 0.05 for the TE and TM polarizations, respectively. These characteristics show the suitability of the device for multipeak and narrow bandwidth infrared light source applications.


Fig. 1. Schematic diagrams of device. (tilt view)	Fig. 2. The thermal emission spectra of the device. The black, green and red lines represent the unpolarized, TE and TM polarized radiation spectra.

Fig. 3. The relationship between the peak positions of WMs with TE and TM polarizations as a function of (a) the top SiO₂ thickness, while the bottom SiO₂ layer thickness is fixed at 400 nm, and (b) the bottom SiO₂ layer thickness, while the top SiO₂ layer thickness is fixed at 1400 nm.

Combined Experimental and Theoretical Studies on II-VI Ternary CdSeTe and CdZnTe Alloys

Professor Zhe-Chuan Feng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The optical, structural, and electrical properties of luminescent II-VI compound semiconductors with bandgap energies ranging from 0 to 4 eV are appealing for ultrasensitive multiplexing/multicolor applications in a variety of emerging areas of biotechnology, nanoscale optoelectronics, and nanophotonics. By varying the composition and controlling the lattice constants in ternary or quaternary alloys, we can achieve greater flexibility of tuning emission and absorption wavelengths for high-efficiency solid-state light emission sources. Earlier, the applications of II-VI materials for photonic devices were hampered primarily by the availability of poor-quality crystals and the difficulty of managing doping. Progress in the modern crystal growth techniques such as metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) etc., has offered higher quality and greater versatility in the preparation of thin films with controlled doping on many convenient substrates.

CdSe_xTe_1-x is the II-VI-VI ternary compound, possesses a zinc-blende structure for x<0.36. The ability to prepare zinc–cadmium (mercury)–based binary [AB, with lended A = Zn, Cd, and Mn (Hg) and B = S, Se, and Te] compounds and thin films of ternary A_1−xB_xC (e.g., Cd_1−xZn_xTe, CdTe_1−xSe_x, etc.) or quaternary A_1−x−yB_xC_yD (e.g., Cd_1−x−yZn_xMn_yTe, CdSe_xS_yTe_1−x−y , etc. where C and D can be the elements of the binary compound AB) alloys with precise chemical compositions x, y has now opened up many possibilities of using II-VI materials in various technological applications.

We have measured far-infrared (FIR) reflectance spectra for CdSeTe and CdZnTe alloys, as shown above, and in collaboration with Prof. Devki Talwar, an excellent theorist, also theoretical calculations on their phonon dispersions, as shown below [1]. Further, to II-VI compound semiconductors, we have used a comprehensive Green’s function theory to study the vibrational properties of isotopic defects and to ascertain the microstructure of complex centers involving dopants and intrinsic impurities. [2]

[1] Devki N Talwar, Tzuen-Rong Yang, Zhe Chuan Feng and P. Becla, “Infrared reflectance and transmission spectra in II-VI alloys and superlattices”, Physical Review B 84, 174203 (2011).
[2] Devki N. Talwar, Zhe Chuan Feng and Tzuen-Rong Yang, “Vibrational signatures of isotopic impurities and complexes in II-VI compound semiconductors”, Physical Review B 85, 195203 (2012).

Determination of Surface Plasmon Modes and Guided Modes Supported by Periodic Subwavelength Slits on Metals Using a Finite-Difference Frequency-Domain Method Based Eigenvalue Algorithm

Professor Hung-chun Chang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An eigenvalue solution algorithm is formulated based on the finite-difference frequency-domain (FDFD) method for determining guided modes, including the surface plasmon modes, supported by periodic metallic structures. The Yee-mesh grids which have been popularly adopted in the finite-difference time-domain (FDTD) method are used in the FDFD method and standard eigenvalue matrix equations are obtained for easily searching for the guided eigenmodes. Both two-dimensional (2-D) and three-dimensional (3-D) structures are considered and the periodicity is along the propagation direction. The metals are assumed to be perfect ones or real ones without loss. For 2-D structures, an array of grooves drilled in a perfect conductor and a real-metal structure with a periodic arrangement of subwavelength slits in air are analyzed and the dispersion diagrams and mode-field profiles are obtained. For the latter structure, surface plasmon modes and dielectric slab modes are identified to be in agreement with published results based on a different numerical scheme. This subwavelength-slit structure is then extended to a 3-D one having an additional depth and it is demonstrated that the formulated algorithm can solve the same two kinds of modes for the more complicated 3-D problem. The modes guided along drilled periodic rectangle holes on a perfect conductor surface are also calculated. (IEEE/OSA Journal of Lightwave Technology, vol. 30, no. 1, pp. 76–83, 1 January 2012.)

Fig. 1. (a) Schematic of the 3-D structure generalized from the 2-D periodic arrangement of subwavelength slits. (b) Schematic of the three sides of one metallic block in the x-z, x-y, and y-z planes.

Fig. 2. Dispersion diagram of the guided modes on the structure of Fig. 1. The red solid lines represent surface plasmon modes and the blue solid lines represent a series of effective dielectric slab modes. The right panel is the expanded view for more clear visualization of the surface plasmon modes.

Fig. 3. E_x field profiles at k_x = p/d on the three sides of one metallic block, as plotted in Fig. 1(b), i.e., in the x-z, x-y, and y-z planes, for each of the four suface plasmon modes mode in Fig. 2. (a) Mode (1). (b) Mode (2). (c) Mode (3). (d) Mode (4).

Regularly Patterned InGaN/GaN Quantum-well Nanorod Light-emitting Diode Arrays

Professor C. C. (Chih-Chung) Yang's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

With the nano-imprint lithography and the pulsed growth mode of metalorganic chemical vapor deposition, a regularly-patterned, c-axis nitride nanorod (NR) array of quite uniform geometry with simultaneous depositions of top-face, c-plane disc-like and sidewall, m-plane core-shell InGaN/GaN quantum well (QW) structures is formed. The differences of geometry and composition between these two groups of QW are studied with scanning electron microscopy, cathodoluminescence, and transmission electron microscopy (TEM). In particular, the strain state analysis results in TEM observations provide us with the information about the QW width and composition. It is found that the QW widths are narrower and the indium contents are higher in the sidewall m-plane QWs, when compared with the top-face c-plane QWs. Also, in the sidewall m-plane QWs, the QW width (indium content) decreases (increases) with the height on the sidewall. The observed results can be interpreted with the migration behaviors of the constituent atoms along the NR sidewall from the bottom.


Fig. 1 Plan-view (a) and 30^o-tilted (b) SEM images of the GaN NR array; Plan-view (c) and 30^o-tilted (d) SEM images of the QW NR array.	Fig. 2 Plan-view SEM (a) and the co-located panchromatic CL (b) images of the QW NR array; Cross-sectional SEM (c) and the co-located panchromatic CL (d) images of the QW NR array. The rectangles in (a) and (c) indicate the locations of local CL spectrum measurements.

Fig. 3 CL spectra of the QW NR array measured at different locations and different view directions, including that from the large-scale plan-view (PV) measurement, that at the center on the top face of an NR (TF-c) and that at the rim on the top face of the NR (TF-r), that from the large-scale cross-sectional view (CS) measurement, that at a point near the top of the sidewall (SW-t), near the middle height of the sidewall of the NR (SW-m), and near the bottom of the sidewall of the NR (SW-b). The plan-view CL spectrum of the bare GaN NR array is also plotted as curve PV-GaN.

Fig. 4 (a) Cross-sectional TEM image of a QW NR. The portions of the top, the slant (1-101) facet on the right, the top sidewall on the right, the middle-height sidewall on the left, and the bottom sidewall on the right of the NR are magnified to show parts (b) and (d)-(g), respectively. The HAADF image of the NR top portion is shown in part (c).