Cheng-Tien Wan

Three-Band White Light-Emitting Diodes Based on Hybridization of Polyfluorene and Colloidal CdSe–ZnS Quantum Dots

The authors have demonstrated the hybridization of polyfluorene (PFO) polymer light-emitting diodes with CdSe-ZnS core/shell quantum dots (QDs) in different device structures. To achieve white light emission, the green and red QDs have been incorporated into the PFO as the single emissive layer. Furthermore, the whole structures were also optimized to engineer the emission spectra. Accordingly, the incorporation of QDs increased the turn-on voltage from 10 to 23 V in device with the blend ratio of PFO : green QD : red QD being 6 : 1 : 1, while the maximum brightness was dramatically decreased.

High performance Ge CMOS with novel InAlP-passivated channels for future sub-10 nm technology node applications

We report the first realization of high performance Ge CMOS using a novel InAlP passivation scheme. The large conduction band and valence band offsets between InAlP and Ge confine electrons and holes within the Ge channel for n-FETs and p-FETs, respectively. The InAlP cap reduces scattering due to high-K/InAlP interface traps and boosts carrier mobility. As a result, a record high electron mobility μ_EFF of ~958 cm²/V·s at N_INV of 6×10¹¹ cm^-2 was achieved for Ge(100) n-FETs, and a high peak hole mobility of ~390 cm²/V·s was obtained for Ge(100) p-FETs. High on-state currents I_ON of 39.5 μA/μm and 31.2 μA/μm were achieved at gate overdrive |V_GS-V_TH| = 1 V and |V_DS| = 1 V for the n-FETs and p-FETs, respectively, with a gate length L_G of ~3 μm. In addition, for the first time, this novel InAlP passivation technique was integrated into Ge n-FinFETs, and good control of short channel effects (SCEs) was achieved.

Performance Improvement of InGaAsN/GaAs Quantum Well Lasers by Using Trimethylantimony Preflow

The performance characteristics of InGaAsN quantum well (QW) lasers with and without trimethylantimony (TMSb) preflow have been studied. The TMSb preflow before the growth of InGaAsN QWs can suppress the Al-contamination effect and decrease the threshold current density compared with conventional InGaAsN QW lasers without preflow. The photoluminescence (PL) intensity increased and linewidth decreased when TMSb flow rate increased. According to the atomic force microscopy (AFM) measurement, the surface roughness was also reduced significantly after TMSb treatment which manifested that the preflow prevented the Al and N precursors from reacting with each other and resulted in a higher optical quality in InGaAsN QWs.

Low Transparency Current Density and Low Internal Loss of 1060-nm InGaAs Laser With GaAsP–GaAs Superlattices as Strain-Compensated Layer

In this letter, the strained In_0.22Ga_0.78As-GaAs single quantum-well lasers grown by metal-organic vapor phase epitaxy were studied. The lasing wavelength of the fabricated InGaAs laser was 1056 nm, whereas the internal loss (alpha_i) and the transparency current density (J_tr) were 1.78 cm^-1 and 40.2 A/cm², respectively. By using the GaAsP-GaAs superlattices as strain-compensated layer, the lasing wavelength was 1052 nm, and the alpha_i and J_tr could be reduced to 0.63 cm^-1 and 39.1 A/cm², respectively. To the best of our knowledge, the J_tr was the lowest among the reported InGaAs lasers around 1060 nm.

Improvement of defect reduction in semi-polar GaN grown on shallow-trenched Si(001) substrate

The improved design of sub-micron trenches on Si(001) substrate was demonstrated for defect suppression in semi-polar selectively-grown GaN layers. Cathodoluminescence and transmission electron microscopy measurements revealed a dramatically decreased density of threading dislocations and stacking faults near the surface of the overgrown GaN layer when the trench width ranged from 500 to 1500 nm. It was observed that defects were effectively trapped inside the trench when the ratio of trench depth to the SiO2 thickness is less than 0.66. In addition, a significant reduction of intrinsic polarization electric field was achieved for the InGaN/GaN multiple quantum well on the GaN selectively grown from the Si trenches.

Hybrid Quantum Dot Light-Emitting Diodes: Design, Fabrication, and Characterization

The authors have designed and fabricated the colloidal quantum dot (QD) light-emitting diodes based on a thin CdSe/ZnS QD layer sandwiched by organic hole and electron transport layers. The device configuration can be depicted as ITO/PEDOT:PSS/poly-TPD/QD/BCP/Alq3/LiF/Al. The optimized light turn-on voltage is about 7 V and the maximum brightness is 141 cd/m2 at 11.5 V. It has been observed the second organic layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), not only served for the electron transporting but also for hole blocking. It can greatly improve the device performance by enhancing the probabilities of electron and hole injection into the QDs. In device characterization, room temperature photoluminescence (PL) and electroluminescence (EL) peaks are at 625 and 628.6 nm, respectively. The red-shift may be from the local Joule heating induced by the injection current. Emission from Alq3 at 516 nm is also observed in all EL spectra. Specifically, the QD-dominant EL spectra indicate the QD emission is mainly from the direct injection and radiative recombination of carriers rather than the radiative and nonradiative (Forster) energy transfers from excitons in organic materials shown in single-layered hybrid devices. Since the emission spectra are dominated by the inorganic QDs, the improvement of reliability of hybrid devices can thus be expected. However, the luminance efficiency (~0.014 cd/A) is still not as high as that of pure organic or polymer LEDs. To increase the brightness and efficiency, the device structure has to be modified and equivalent injection of electrons and holes is necessary.

Characterization of quantum-well and super-lattice lasers

In this paper, we made an in-depth study on the fabrication of low-pressure MOCVD-grown 1055-1064 nm multi-quantum-well and super-lattice laser diodes, which can be used for green-light generation combined with second-harmonic generation crystal. Due to the simplicity of the configuration and high reliability, large merits in terms of size, cost, and power consumption would arise. Furthermore, direct modulation is possible in the laser diodes, which would lead to a highly compact pulsed laser source.

Hybridization of CdSe/ZnS Quantum Dots on InGaN/GaN Multiple Quantum Well Light-Emitting Diodes for Pink Light Emission

Pink light emission has been demonstrated from the hybrid CdSe/ZnS quantum dot-InGaN/GaN quantum well light-emitting diodes (LEDs). The QDs in toluene are blended in the resin matrix to become the nanophosphor. For the fabricated hybrid LED, it is found the light output power is greatly reduced due to the poor quantum yield (QY) of QDs (<50%). Besides, the comparison of the electroluminescence (EL) spectra of the InGaN blue LED and the hybrid pink LED exhibit over 90% of the blue light centered at 448 nm has been down-converted to the red light at 636 nm. Consequently, purplish pink light with CIE-1931 chromaticity coordinates of (0.374, 0.147) is obtained. Also observed is the significant blue-shift of luminescence peak from QD-resin composite with respect to the photoluminescence peak from QD-toluene solution, which is due to the less attack of reabsorption of photon energies from smaller to larger QDs rather than the Forster energy transfer. The current-dependent and temperature-dependent EL spectra are also characterized to evaluate the thermal stability of our hybrid LED. In this demonstration, CdSe/ZnS QDs with different sizes/colors can be used as nanophosphors to fabricate color-converted LEDs. Nevertheless, QDs with higher QYs or more efficient device packagings should be adopted to improve the device efficiency.

Optical Studies of GaAs Nanowires Grown on Trenched Si(001) Substrate by Cathodoluminescence

The strains in GaAs nanowires, which were grown from 1700- to 80-nm-wide trenches on the Si(001) wafer with SiO2 masks, were investigated by cathodoluminescence. For 1700- to 500-nm-wide trenches, the in-plane tensile strain at 15 K decreases with the decreasing trench width. The strain increases abruptly when the trench width is 300 nm, and then decreases as the trench width is further decreased. The results revealed that the stress induced by the SiO2 sidewalls dominates when the width is less than the depth of the trench. This approach provides an effective technique to measure the strain of a single nanowire and helps for the demonstration of selectively-grown GaAs with a designed strain.

An in-depth study on the fabrication of 1055–1064 nm multi-quantum-well and super-lattice laser diodes

The InGaAs/GaAs multi-quantum-well as well as the AlGaAs/GaAs and the GaAsP/GaAs super-lattice laser diodes were successfully fabricated by low-pressure MOCVD system, and a number of novel structures were explored in this systematic investigation. The strain-relief effect and the composition of cladding layers were analyzed in detail. Via a series of growth experiments, we concluded that better lasing efficiency and the minimum threshold current could be obtained from the sample made up of the AlGaAs/GaAs structure combined with the 60% Aluminum content of the AlGaAs cladding layer.