GIPO newsletter

Temperature dependence of Raman scattering in bulk 4H-SiC with different carrier concentration

Professor Zhe-Chuan Feng's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Raman spectra of three bulk 4H-SiC wafers with different free carrier concentration were measured in 80-873K. As temperature increases, Raman peaks of most optical phonon modes show monotonous down shift. An anomalous non-monotonous variation with temperature, was observed in the A₁ longitudinal optical (LO) mode from doped samples. Two methods of theoretical fitting, one-mode (LO-plasma coupled (LOPC) mode) and two-mode (A₁(LO+LOPC) fitting, are employed to analyze this anomalous phenomenon. Theoretical simulations for temperature dependent Raman spectra using two methods are critically examined.

Figure 1. Raman spectra of three 4H-SiC samples with different doping levels at temperature varying from 80 K to 873 K.

Figure 2. Theoretical fitting results of E₁(TO) and E₂(TO) mode peak positions of three 4H-SiC samples.

Figure 3. Temperature dependence of (a) plasma frequency ω_p (b) experimental results (solid line) and theoretical calculation (short dot line) of LOPC mode peak positions, in one-mode method.

Confocal micro-Raman spectroscopy measurements at temperature varying from 80 K to 873 K and theoretical analyses on bulk 4H-SiC wafers, undoped, medium doped and high doped, were done. Down shift of E₂ (TO) and E₁ (TO) phonon scattering modes is explained through phonon frequency function with temperature. Anomalous variation of A₁(LO) mode in doped 4H-SiC stems from the coupling between LO phonons and plasma, which forms the LOPC mode. Two methods, one-mode and two-mode methods, are applied to analyze variation of A₁(LO) mode. These two method fits have appeared in the literature for many years but no judgment on which one being more reliable was given yet so far. We have demonstrated clearly that the two-mode method is untenable in interpreting anomalous variation of A₁(LO) mode with temperature, while the one LOPC mode simulation is in good accordance with experimental results. The non-monotonous variation of blue-red shifts with temperature for LOPC mode from doped 4H-SiC could be explained by the influence from ionization process of impurities on the process of Raman scattering. A quantitative description on temperature dependent Raman spectra for doped 4H-SiC is achieved in this paper, matching well to experimental data.

Optics Express, Vol. 21, no. 22, 26475–82 (2013).

The application of IR radiation in biotechnology and cancer therapy

Professor Si-Chen Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have developed different types of innovative narrow bandwidth infrared (IR) radiation emitters, including tri-layers Au/SiO₂/Au waveguide thermal emitter (WTE) and tri-layers Ag/SiO₂/Ag plasmonic thermal emitter (PTE), theirs emission wavelengths were varied from 3 to 10 μm [1-3]. We used the narrow band IR emitters to irradiate the E. Coli and lung cancer cells and investigate its effect. In 2011, ” The effect of narrow bandwidth IR radiation on the growth of Escherichia coli,” was published in in Applied Physic letter [4]. The diameters of E. Coli were affected by IR radiation for 24 hours with PTE peak wavelengths at 3, 3.5, 4, 4.5, and 5 μm, but not other wavelength as shown in Fig. 1. It was identified to be caused by the increased expression of the outer membrane protein OmpA and OmpF, implies nutrition transport and metabolic pathways of E. Coli were significantly regulated by narrow bandwidth IR radiation.

Furthermore, We have cooperated with Prof. Juan (Department of life science, NTU) and found that 3~5 μm IR radiation can induce the A549 lung cancer cell swelling, inhibit the phosphorylation of cyclin B1 and the arrest of G2/M cell cycle progression as shown in Fig. 2. [5]. This study also exhibited that IR radiation triggered ATM/ATR-p53-p21 axis in response to DNA damage, leaded to the formation of 53BP1 and g-H2AX nuclear foci, implies that IR radiation induced the DNA repair system to fix the DNA damage. Additionally, we have investigated that IR radiation with WTE peak wavelengths at 4.1, and 5.0 μm can induce the collapse of mitochondrial membrane potential of HeLa cervical cancer cells to enhance the therapeutic effects of Paclitaxel.

In summary, The IR radiation could be used to enhance the growth of E. Coli for biotech application, or combined with chemotherapy to treat cancer cells. This technology has potential to be applied in clinical cancer therapy in the future.


Figure 1. The colony diameters of E.coli treated with different wavelengths of PTE for 24 hours [5].	Figure 2. The genes related to G2/M transition of A549 cells treated with the 3~5 μm IR radiation (MIR) for 48 hours [6].

References
1. Wu, Y.-T., et al., Narrow Bandwidth Midinfrared Waveguide Thermal Emitters. Photonics Technology Letters, IEEE, 2010. 22(15): p. 1159-1161.
2. Tsai, M.-W., et al., High performance midinfrared narrow-band plasmonic thermal emitter. Applied Physics Letters, 2006. 89(17): p. 173116-3.
3. Chen, H.-H., et al., Two infrared emission modes with different wavelengths and orthogonal polarization in a waveguide thermal emitter. Journal of Applied Physics, 2012. 112(7): p. 074325-5.
4. Tsai, S.-R., et al., The effect of narrow bandwidth infrared radiation on the growth of Escherichia coli. Applied Physics Letters, 2011. 99(16): p. 163704.
5. Chang, H.Y., et al., Middle infrared radiation induces g(2)/m cell cycle arrest in a549 lung cancer cells. PLoS One, 2013. 8(1): p. e54117.

Simulating light transmission through a metallic thin film perforated with 2D periodic array of multiple-slit apertures

Professor Hung-chun Chang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The phenomenon of extraordinary transmission through periodically arranged sub-wavelength holes in metal films has attracted considerable interest during past decades. The main mechanisms responsible for this phenomenon have been widely believed to be the excitation of surface plasmon polaritons (SPPs), or the surface electromagnetic (EM) modes, and Wood’s anomalies (WAs) set up by periodically arranged holes. In addition, it has been shown that the shape of the aperture, resulting in shape resonances (SRs), has a strong effect on the transmission spectra. In this work, we study the optical properties of differently shaped apertures composed of one to three horizontal slits, named x-arm slits, and one vertical slit, named y-arm slit. Figure 1(a) shows the geometry of a unit cell of one sub-wavelength metallic structure example we study. The metal (silver) film with thickness 75 nm is coated on a silicon substrate. An in-house developed 3D finite-difference time-domain (FDTD) method numerical model is used to simulate the EM field distributions and transmission spectra under normal incidence of light. Periodic boundary conditions are applied to the xz and yz planes to simulate a square periodic array of apertures. The SR wavelength might dominantly depend on the slit length. But for more complex-shaped apertures, the SR paths may have some ambiguities to be estimated by the modified cutoff wavelengths of the rectangular waveguide (i.e., λ_res = 2n_effL_res/m, where is the effective refractive index, L_res is the resonant length, and m is an integer) due to the slight difference of the L_res between those possible contours. We have found that analyzing the near-field features is helpful in distinguishing the SR modes. For example, the modulus of the electric field, |E|, defined by , contours clear geometry of the resonant length. Within the SR paths, the distribution of the phase component parallel to the polarization of the incident light (e.g., φ_Ex under x-polarized light) is quite homogeneous and the phase component along the propagation direction, φ_Ez, exhibits a 180^。jump at the center of the slit width. By focusing on the anti-phase property of φ_Ez along the SR contours and the symmetry requirement preserved for certain polarization state of light, those SR paths that cannot meet the above two criteria together are obviated and the possible right ones are left. In order to testify the validity of such principles in determining the SR paths, we design a set of apertures with onefold or twofold mirror symmetry, composed of x-arm slits and one y-arm slit, as shown in Fig. 1(b). We find the FDTD-obtained field-profile predictions agree well with the real SR modes, which are validated by the near-field analyses. Moreover, we discuss how the separation distance between slit elements in the apertures affects the SR wavelength under specific incident-wave polarization, showing how the coupling between adjacent slits would play a role in the variation of the spectra, as shown in Fig. 1(c). By delicately studying the properties of such factors and how they interact, we could manipulate the spectra with an additional degree of freedom, which could be important to structures with multi-holes or multi-slits in one unit cell. (Hui-Hsin Hsiao and Hung-chun Chang, in META’13 Abstracts, pp. 161–162, March 18–22, 2013.)

(a)	(b)	(c)
Figure 1. (a) The unit cell and the near-field plane cut in our simulation. (b) Top view and structural parameters of samples 1–5. (c) Transmission spectra with an y-polarized light under normal incidence. The inset shows two adjacent unit cells with separation distances between the x-arm slits, s₁ and s₂, specified.

Strain reduction and crystal improvement of an InGaN/GaN quantum-well light-emitting diode on patterned Si (110) substrate

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

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The comparisons of the morphology, material property, and optical characteristics of a crack-free InGaN/GaN quantum-well light-emitting diode (LED) structure of completely coalesced overgrowth on an a-axis-oriented one-dimensional trench-patterned Si (110) substrate with the other two samples grown on flat Si (110) and Si (111) substrates are demonstrated. This sample on patterned Si (110) substrate shows the highest crystal quality, weakest tensile strain, largest internal quantum efficiency, strongest LED output intensity, lowest device resistance, and smallest spectral shift range in increasing injection current. The small spectral shift range indicates the weak quantum-confined Stark effect. The advantages of this sample are attributed to the small lattice mismatch between Si and GaN along the m-axis, the reduced thermal stress along the m-axis, and the minimized upward-propagating dislocation density. Figure 1 shows the cross-sectional scanning electron microscopy (SEM) images of the sample with patterned Si (110) substrate (sample A). One can see the preserved trenches in the Si substrate. Figure 2 shows the plan-view SEM images of the three samples: (a) patterned Si (110) substrate-sample A, (b) flat Si (110) substrate-sample B, and (c) flat Si (111) substrate-sample C. Only sample A is crack-free. Figure 3 shows the Raman scattering spectra of the three samples. Sample A is strain free. Figure 4 shows the LED output intensities (left ordinate) and spectral peak energies (right ordinate) as functions of injection current of the three samples. Sample A has the highest output intensity and the smallest spectral shift.

Fig. 1 Cross-sectional SEM images of the sample with patterned Si (110) substrate.

Fig. 2 Plan-view SEM images of the three samples: (a) patterned Si (110) substrate, (b) flat Si (110) substrate, and (c) flat Si (111) substrate.


Fig. 3 Raman scattering spectra of the three samples.	Fig. 4 LED output intensities (left ordinate) and spectral peak energies (right ordinate) of the three samples.

Surface plasmon coupling with a radiating dipole near an Ag nanoparticle embedded in GaN

Professor Y. W. Kiang’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The localized surface plasmon (LSP) coupling behaviors of a radiating dipole with an Ag nanosphere (NS) embedded in GaN are evaluated based on a numerical algorithm, which takes account of the induction of the LSP resonance on the Ag NS by the radiating dipole and the feedback of the LSP resonance to the radiating behavior of the dipole. In particular, we compute the enhancements of dipole strength and radiated power through such a coupling process for the radiating dipoles oriented along the radial and orbital directions with respect to the Ag NS. The enhancements of radiated power of a radial and an orbital dipole are induced through the coupling with the LSP dipole and higher-order resonance, respectively. These results are interpreted with the interference behaviors between the radiations of the source dipole and the LSP mode of the Ag NS.

Fig. 1. Absorption cross sections as functions of wavelength of an Ag NS of various radii at R₀ = 2.5, 5, 10, 15, and 20 nm. The insert shows the geometry of the LSP coupling system, including an embedded Ag NS with radius R₀ centered at the coordinate origin and a radiating dipole located at (x, 0, a), which is represented by a thick (pink) arrow.

Fig. 2. Normalized radiated powers (Rad.) as functions of wavelength in the case of a radial dipole for d = 30 nm. The normalized absorbed power (Abs.) as a function of wavelength for R₀ = 20 nm is also plotted.

Properties and device application of GaAsPSb lattice matched to GaAs

Professor Hao-Hsiung Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

GaAsPSb is an alloy consisting of three binaries; GaP, GaAs, and GaSb. The bonds of the three binaries are highly mismatched in length. As a result, the bonds undergo strong stretching and bending when they are packed into the alloy lattice. Although the strong bond distortion leads to difficulty in epitaxial growth, the distortion, especially the bending, strongly affects the electronic band structure. Theoretical calculation showed that the randomly bending increases the bowing of the valence band. This property has been studied using photoreflectance spectroscopy and applied to heterojunction bipolar transistor (HBT) with low turn-on voltage. Fig. 1 shows the transition energies of conduction band to heavy hole, light hole and spin orbit bands of GaAs_0.64P_0.19Sb_0.17. From the transition energies, we found an unusual large spin orbital splitting of 0.363 eV, which is attributed to the bowing of the valence band resulting from bond distortion. Because of the upward valence bowing, the alloy and GaAs are in type-II band lineup. We have fabricated a HBT with p⁺-GaAs_0.57P_0.28Sb_0.15 base. Thanks to the type-II band alignment, the device shows a reduction of turn-on voltage by 70 mV, a current gain of 40, and a very low knee voltage up to J_C = 40 kA/cm². The Gummel plot and output I-V characteristics of the HBT are shown in Fig. 2 and 3, respectively.


Fig. 1 Conduction band to heavy hole band, light hole band, and spin orbit band transition energies of GaAs_0.64P_0.19Sb_0.17 as functions of temperature.	Fig. 2 Gummel plots of the control InGaP/ GaAs SHBT and InGaP/GaAs_0.57P_0.28Sb_0.15 DHBT. The hollow dot is the I_c of the reverse Gummel plot of the InGaP/GaAsPSb DHBT.

Fig. 3 Common emitter output characteristics of HBT with GaAsPSb base and HBT with GaAs base.