Tung-Wei Chi

Engineering laser gain spectrum using electronic vertically coupled InAs-GaAs quantum dots

Continuous large-broad laser gain spectra near 1.3 /spl mu/m are obtained using an active region of electronic vertically coupled (EVC) InAs-GaAs quantum dots (QDs). A wide continuous electroluminescence spectrum, unlike that from conventional uncoupled InAs QD lasers, was obtained around 230 nm (below threshold) with a narrow lasing spectrum. An internal differential quantum efficiency as high as 90%, a maximum measured external differential efficiency of 73% for a stripe-length of L=1 mm, and a threshold current density for zero total optical loss as low as 7 A/cm/sup 2/ per QD layer were achieved.

A Novel Dilute Antimony Channel

This letter reports, for the first time, a high-electron mobility transistor (HEMT) using a dilute antimony In0.2Ga0.8 AsSb channel, which is grown by a molecular-beam epitaxy system. The interfacial quality within the InGaAsSb/GaAs quantum well of the HEMT device was effectively improved by introducing the surfactantlike Sb atoms during the growth of the InGaAs layer. The improved heterostructural quality and electron transport properties have also been verified by various surface characterization techniques. In comparison, the proposed HEMT with (without) the incorporation of Sb atoms has demonstrated the maximum extrinsic transconductance gm,max of 227 (180) mS/mm, a drain saturation current density IDSS of 218 (170) mA/mm, a gate-voltage swing of 1.215 (1.15) V, a cutoff frequency fT of 25 (20.6) GHz, and the maximum oscillation frequency fmax of 28.3 (25.6) GHz at 300 K with gate dimensions of 1.2times200 mum2

\hbox{In}_{0.2}\hbox{Ga}_{0.8}\hbox{AsSb}/\hbox{GaAs}

Strain relaxation in InAs/InGaAs quantum dots (QDs) is shown to introduce misfits in the QD and neighboring GaAs bottom layer. A capacitance?voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. The emission-time increase is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with observed emission energies of 0.160 and 0.068?eV which are comparable to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60?meV is observed, corresponding to the emission from the QD ground state to the first-excited state.

HEMT

Extended microtunnels with triangular cross sections in thick GaN films were demonstrated using wet chemical etching on specially designed epitaxial lateral overgrowth structures. For tunnels along the and directions of GaN, the {1122} and {1011} facets are the etch stop planes with activation energies of 23 kcal/mol determined by wet chemical etching. The axial etching rate of the tunnels in the (1100) direction is twice as large than that along the (1120) direction. The highest etching rate of the tunnels in the axial direction is 1000 μm/h.

Relaxation-induced lattice misfits and their effects on the emission properties of InAs quantum dots

This work reports for the first time a novel In0.2Ga0.8AsSb/GaAs heterostructure doped-channel field-effect transistor (DCFET) grown by the molecular beam epitaxy system. The interfacial quality within the InGaAsSb/GaAs quantum well of the DCFET device has been effectively improved by introducing surfactant-like Sb atoms during the growth of the Si-doped InGaAs channel layer. The improved device characteristics include the peak extrinsic transconductance (gm, max) of 161.5 mS mm−1, the peak drain–source saturation current density (IDSS, max) of 230 mA mm−1, the gate–voltage swing (GVS) of 1.65 V, the cutoff frequency (fT) of 12.5 GHz and the maximum oscillation frequency (fmax) of 25 GHz at 300 K with the gate dimensions of 1.2 × 200 µm2. The proposed design has also shown a stable thermal threshold coefficient (∂Vth/∂T) of −0.7 mV K−1.

Triangular Extended Microtunnels in GaN Prepared by Selective Crystallographic Wet Chemical Etching

Quantum dots (QDs) have great potential in optical fiber communication applications were widely recognized. The structure of molecular beam epitaxy (MBE) grew InAsN QDs were investigated by transmission electron microscopy (TEM) and measured their optical properties by photoluminescence (PL). TEM images show that the InAsN QDs are irregular or oval shaped. Some of the InAsN QDs are observed to have defects, such as dislocations at or near the surface in contrast to InAs QDs, which appear to be defect free. PL results for InAsN QDs showed a red-shifted emission peak. In addition, the InAsN emission peak is broader than InAs QDs, which supports the TEM observation that the size distribution of the InAsN QDs is more random than InAs QDs. The results show that the addition of nitrogen to InAs QDs leads to a decrease in the average size of the QDs, bring changes in the QDs shape, compositional distribution, and optical properties.

Investigations on highly stable thermal characteristics of a dilute In0.2Ga0.8AsSb /GaAs doped-channel field-effect transistor

Two different template structures of dot air-bridges and nanorods were used for 300 µm GaN growth by hydride vapor phase epitaxy (HVPE). The selective growth of arrays of dot air-bridges and nanorods whose sidewalls coated with SiO2 are identified and exploited to form a compliant layer to decouple the impact due to the different thermal expansion and lattice mismatch between 300 µm overgrown GaN layer and the host sapphire substrate. As the process temperature cooling down from 1050 °C to room temperature in HVPE system, the 300 μm freestanding GaN substrates were obtained by the self-separation. The dislocation density was estimated by both the etching pit density method and cathodoluminescence (CL). The dislocation densities of the freestanding bulk GaN were lower than 5×106 and 5×107 cm-2 for the template structure of dot air-bridges and nanorods structure, respectively.

Analysis of InAsN quantum dots by transmission electron microscopy and photoluminescence.

A 220-µm-thick Gallium nitride (GaN) layer was homoepitaxially regrown on the Ga-polar face of a 200-µm-thick free-standing c-plane GaN by hydride vapor-phase epitaxy (HVPE). The boundary of the biaxial stress distribution in the GaN substrate after regrowth was clearly distinguished. One half part, the regrown GaN, was found to be more compressive than the other half part, the free-standing GaN. Additionally, the densities of the screw and mixed dislocations reduced from 2.4 ×107 to 6 ×106 cm-2 after regrowth. Furthermore, the yellow band emission almost disappeared, accompanied by a peak emission at approximately 380 nm related to the edge dislocation was under slightly improved in regrown GaN. We conclude that the reduction of the dislocation defects and Ga vacancies and/or O impurities are the two main reasons for the higher compressive stress in the regrown GaN than in the free-standing GaN, causing the curvature of the GaN substrate to be twice concave after regrowth.

Comparison of different template structures for high quality and self-separation thick GaN growth

A novel In0.3Ga0.7As0.99N0.01(Sb)/GaAs high-electron-mobility transistor has been successfully investigated for the first time by incorporating surfactant Sb atoms during the InGaAsN channel growth by molecular beam epitaxy (MBE). Superior stable thermal characteristics, including a thermal threshold coefficient (∂Vth/∂T) of -0.807 mV/K and a high-temperature linearity (∂GVS/∂T) of -0.053 mV/K, were achieved because of the improved crystalline quality and the enhanced carrier confinement capability of the In0.3Ga0.7As0.99N0.01(Sb)/GaAs heterostructure. The device also demonstrated a peak extrinsic transconductance (gm,max) of 94 (109) mS/mm at 450 (300) K.

Stress and Defect Distribution of Thick GaN Film Homoepitaxially Regrown on Free-Standing GaN by Hydride Vapor Phase Epitaxy

Transistor with a Novel In0.3Ga0.7As0.99N0.01(Sb) Dilute Channel Ke-Hua Su, Wei-Chou Hsu, Ching-Sung Lee,Yu-Shyan Lin, Po-Jung Hu, Ru-Shang Hsiao, Tung-Wei Chi and Jim-Y Chi Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan 70101, R.O.C. TEL: 886-6-2757575-62303/FAX: 886-6-2094786 *Corresponding author. E-mail:wchsu@eembox.ncku.edu.tw