GIPO newsletter

Environmentally Benign Technology for White Lighting

Professor Ching-Fuh Lin

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

White light-emitting diodes (WLEDs) have the great potential to make a tremendous impact on energy consumption throughout the world. Unfortunately, conventional WLEDs phosphors utilize expensive rare-earth elements (REEs) and the rare-earth mining, refining and disposal bring extremely high environmental cost. Therefore, the development of eco-friendly phosphors free of REEs and toxic metals has been one of the challenges for the next-generation light sources. Although quantum dots (QDs) and nanocrystals (NCs) have been considered as promising alternatives, the strong quantum confinement effect in these nanoscale materials causes their efficient excitation wavelength to be limited in UV region. This limitation severely impedes their integration with commercial blue LEDs to fabricate high luminous efficacy WLEDs.

Therefore, we use II-VI nanoparticle engineering to serve as an environmentally responsible strategy for white light generation. We successfully develop a novel technique to synthesize II-VI nanoparticles without quantum-confinement effect. Because the photo-physical behavior is not restricted by quantum confinement, the nanoparticles can exhibit a strong absorption at 453 nm, which well matches the wavelength of commercial blue-LEDs (450-460 nm). Also, the nanoparticles can efficiently convert blue light (440-460 nm) to orange light (580 nm). Integrating with commercial blue-LED (450 nm), the emission of both candle light and bright white light can be clearly observed (Figure 1).

Additionally, we integrate ZnO and ZnS:Mn semiconductor nanoparticles with polymeric materials poly(9,9-di-n- hexylfluorenyl-2,7-diyl). The resultant nanocomposites can be endowed with three different photon-generating mechanisms corresponding to blue, green and orange emssions, respectively. Consequently, white light can be produced and exhibits widely tunable color temperatures, ranging from 2100 K to 6000 K. As shown in Figure 2, the photoluminescence spectra and the image indicate that the emssion of the nanocomposites has very low color temperature, similiar to candle light, which is good for human health. A warm-white light emission with 90% high quantum efficiency (QE) has been demonstrated under the commercial UV-LED excitation.

The proposed nanotechnology-based materials can not merely serve as a judicious strategy to overcome the health issues involved in current blue-YAG-LED lighting, but also eliminate the use of REEs for environmentally friendly solid-state-lighting technology.

Figure 1. Photoluminescence (PL) excitation and emission spectra of the prepared II-VI nanoparticles.

Figure 2. PL emission spectra of the cold white and warm white nanocomposites.

Lateral Development Rate to Reduce Proximity Effect in e-Beam Lithography

Professor Chieh-Hsiung Kuan

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An ideal strategy for ebeam lithography is to apply the primary beam for exposure. The deposited energy on the resist by primary beam inside this narrow region is relatively extremely higher than the outside forward scattering and backscattering regions and causes a much larger solubility of resist (for positive tone resists). Therefore, the developing rate of resist in primary beam region is much higher than that in outside scattering regions. In this work, we propose a method based on rapid development to separate the primary beam region and electron scattering region and hence to effectively reduce the proximity effect.

Fig. 1 shows the doughnut patterns with various inner radiuses from 100nm to 1.5μm and fixed outer radius of 15μm that were exposed with various dose time from 0.5μs to 15μs. After developing for 5min to ensure complete development, if the resist at the center of pattern was just developed as a collapsed cone, the critical conditions of Ri and dose time were obtained. Fig. 2 shows the fitting curve and the results of EID. The EID displayed the distribution of electrons but not the distribution of electron energy. The high-energy primary beam electrons were integrated inside the forward scattering region and not revealed in EID profile. Fig. 3, the result of single spot experiment shows a rapid increment of hole diameter R in the first few seconds (region 1), and a transfer region (region 2), and finally achieve a steady and slow increment (region 3).

Fig. 1 The diagram and SEM image of doughnut pattern. The shadow region is the scanned region by electron beam.

Fig. 2 (a) The result of doughnut experiment and the fitting curve of equation. (b) The fitting result of EID profile.

Fig. 3 (a) Fitting curves of the single spot experiment with dose time from 15μs to 100 μs. (b) and (c) are two examples of fitting.

High two-dimensional electron mobility in a SiGe/strained Si/SiGe quantum well

Professor Chee Wee Liu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

The two-dimensional electron gas (2DEG) mobility has reached 2 × 10⁶ cm²/V s at 0.3 K for 15 nm strained Si well with a 526 nm Si_0.86Ge_0.14 barrier layer. The SiGe/strained Si/SiGe heterostructure was grown on p-type (001)-oriented Si at 550℃ by hot-walled ultrahigh vacuum chemical vapor deposition. The factors that limit the mobility are the background charges, the threading dislocation density and the roughness at the Si/SiGe heterojunction. Reduction of Ge concentration in the SiGe barrier layer and the relaxed buffer layer that reduces the threading dislocation density and the roughness, effectively enhances the mobility of the 2DEG in the Si channel. The background doping was greatly reduced by high purity gases and the long-time baking of quartz tube.

Figure 1. Electron density dependence of the 2DEG mobility for the sample with Si_0.86Ge_0.14 relaxed buffer layer measured at T = 0.3 K. The maximum value of 2DEG mobility of the Si_0.86Ge_0.14 sample reaches 2 × 10⁶ cm²/V s at the electron density of 1.4 × 10¹¹ cm^-2.

Figure 2. The 2DEG peak mobility as a function of Ge concentration in the relaxed buffer layer. The inset demonstrates the SiGe/Si/SiGe heterostructure schematically.

Threshold voltage controlled by gate area and gate recess in inverted trapezoidal trigate AlGaN/GaN MOS-HEMTs with PEC and PEALD oxides

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

An enhancement-mode, AlGaN/GaN inverted trapezoidal trigate MOS-HEMT wrapped with double-layered oxides, i.e., PEC-grown Ga₂O₃/(Al,Ga)₂O₃ and PE-ALD Al₂O₃, was demonstrated to exhibit positive V_th = 1.2V with current on/off ratio of 10⁸ and sub-threshold swing of 160mV/dec. HRTEM analysis revealed trapezoid-shape channel sidewalls due to crystallographic wet chemical etching by using molten KOH, and a reaction rate-limited process at which the outer surfaces of the nonpolar/semipolar GaN facets are converted into crystalline α-Ga₂O₃ passivation layers with preferential growth of (003) α-Ga₂O₃ upon the c-plane of GaN. From DC characteristics analysis of diverse gate recess depths and different gate areas of fabricated devices, we suspected that there is negative polarization in the interface of double-layer gate oxide and barrier to compensate the strong polarization charge induced by AlGaN/GaN heterostructure, and hence, to lead to higher threshold voltage than other HEMTs without gate oxide. These observations can be ascribed to combined effects of (a) interfacial negative space charge of 3.2 μC/cm² in the gate-recessed device to partially compensate the spontaneous charge, and (b) side-wall passivation to preserve the high mobility channel. It relaxes the constraint of using nanometer scale channel width to achieve positive value of V_th and results in a cut-off frequency of unity current gain/power gain at f_T/f_max = 9/36GHz at gate length/width of L_G /L_w = 250 /360nm.

Fig. 1 (a) Top-view SEM micrograph of inverted trapezoidal trigate AlGaN/GaN MOS-HEMT with gate recess, (b) 3D structural diagram of this device, (c) the f_T and f_MAX plot corresponding to L_G=250nm E-mode AlGaN/GaN inverted trapezoidal trigate MOS-HEMT, and DC measurements for (d) I_DS-V_DS family curves, (e) transfer characteristics and transconductance curve in linear-scale, (f) transfer characteristics of I_DS-V_G in log-scale and I_G-V_G gate leakage current measured in a three-terminal configuration.

Ref: Po-Chun Yeh, Yun-Wei Lin, Yue-Lin Huang, Jui-Hung Hung, Bo-Ren Lin, Lucas Yang, Cheng-Han Wu, Tzu-Kuan Wu, Chao-Hsin Wu, and Lung-Han Peng," Threshold voltage controlled by gate area and gate recess in inverted trapezoidal trigate AlGaN/GaN MOS high-electron-mobility transistors with photoenhanced chemical and plasma-enhanced atomic layer deposition oxides", Applied Physics Express, 8, 084101, 2015.

Broadly tunable and low-threshold Cr⁴⁺YAG crystal fiber laser

Professor Sheng-Lung Huang's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Broadband fiber lasers and amplifiers in the 1.3–1.6 μm wavelength regime, providing extra bandwidth beyond that of the erbium-doped fiber amplifier, could be the key components in future high-capacity telecommunication system. We demonstrate a Cr⁴⁺:YAG crystal fiber laser tunable from 1353 to 1509 nm using a birefringent filter and with a threshold pump power of 70 mW operating at room temperature. It was found that spherical aberration contributed more of the intra-cavity insertion loss of the glass-clad crystal fiber than chromatic aberration. The pump polarization was aligned with a crystal axis to optimize the fiber gain. The advantages of the crystal fiber include the reduced pump threshold due to the light confinement and the better thermal dissipation due to the high surface-area-to-volume ratio. The temperature rise of the crystal fiber with a 400-mW incident pump power was only 1.7 °C using passive cooling. This low-threshold crystal fiber laser is suitable for the application as a tunable seed source for the power amplifier chain.

Figure: Results of the tunable Cr⁴⁺:YAG crystal fiber laser with birefringent filter (BRF). (a) Lasing wavelengths vs. the rotation angle of the BRF. Dashed line: Theoretical tuning curves of the BRF. Dots: Measured laser wavelengths. Background contours: Single-pass TM transmittance of the BRF. (b) Dots: The laser output power vs. laser output wavelengths under 400-mW pump power. Lines: four example spectra lasing at 1357, 1393, 1443, and 1494 nm.