Tzu-Pin Chen

Chirped GaAs-AlAs distributed Bragg reflectors for high brightness yellow-green light-emitting diodes

A novel chirped distributed Bragg reflector (CDBR) structure was grown by metal-organic chemical vapor deposition (MOCVD) on a GaAs substrate where the optical thickness of each pair decreases monotonically from the bottom of the structure to the top surface. It was found that the fabricated CDBR structure can reflect yellow-green light with a maximum reflectivity of more than 80%, and it can reflect light more efficiently in a wider wavelength range than the conventional DBR structure. Yellow-green AlGaInP LEDs with the CDBR structure and the conventional DBR structure were both fabricated. It was found that by using the CDBR structure, one can achieve a higher luminescence intensity.

On an AlGaInP-Based Light-Emitting Diode With an ITO Direct Ohmic Contact Structure

An interesting AlGaInP multiple-quantum-well light-emitting diode (LED) with a direct ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and AuBe diffused thin layer, is fabricated and studied. The direct ohmic contact structure is performed by the deposition of an AuBe diffused thin layer and the following activation process on the surface of a Mg-doped GaP window layer. Experimental results demonstrate that a dynamic resistance of 5.7 Omega and a forward voltage of 1.91 V, under an injection current of 20 mA, are obtained. In addition, the studied LED exhibits a higher external quantum efficiency of 9.7% and a larger maximum light-output power of 26.6 mW. The external quantum efficiency is increased by 26% under the injection current of 100 mA, as compared with the conventional LED without this structure. This is mainly attributed to the reduced series resistance resulted from the relatively uniform distribution of AuBe atoms near the GaP layer surface and the effective current spreading ability by the use of ITO film. Moreover, the life behavior of the studied LED, under a 20-mA operation condition, is comparable to the conventional LED without this structure.

High-brightness AlGaInP 573-nm light-emitting diode with a chirped multiquantum barrier

A novel chirped multiquantum barrier (CMQB) structure was used for AlGaInP light-emitting devices. We have theoretically studied the blocking efficiency of the CMQB structure and found that the CMQB structure is more effective in blocking the electron wave than the conventional uniform multiquantum barrier (UMQB) structure. AlGaInP light emitting diodes (LEDs) with the CMQB structure and the UMQB structure were both fabricated and compared. It was found that the luminescence intensity of the AlGaInP CMQB LED is larger and the intensity distribution of the AlGaInP CMQB LED is more uniform than the AlGaInP UMQB LED. The intensity-current measurement also shows that the electroluminescence intensity of the AlGaInP CMQB LED starts to saturate at a higher injection current.

Study of a New Field-Effect Resistive Hydrogen Sensor Based on a Pd/Oxide/AlGaAs Transistor

A new and interesting field-effect resistive hydrogen sensor, based on the current-voltage characteristics in the linear region of an AlGaAs-based pseudomorphic high-electron-mobility transistor structure and high hydrogen sensitivity of a palladium (Pd) metal, is studied and demonstrated. An oxide layer between Pd and AlGaAs is used to increase the number of hydrogen adsorption sites, and improve hydrogen detection sensitivity. A simple model is employed to interpret the hydrogen adsorption and sensing mechanism. The dissociation of H2, diffusion of H atoms and formation of a dipolar layer cause a significant decrease in channel resistance. In comparison with other resistor-type hydrogen sensors, the studied device demonstrates the considerable advantages of lower detection limit (< 4.3 ppm H2 /air) and higher sensitivity (24.7% in 9970 ppm H2/air) at room temperature. Also, the studied device exhibits a smaller resistance (several 10 Omega) and a smaller operating voltage (les 0.3 V) which are superior to other resistive sensors with typically larger resistances (ranged from kiloohms to megaohms) and larger voltages (ges 1 V). Consequentially, the studied resistive sensor provides the promise for low-power GaAs-based electronic and microelectromechanical-system applications

AlGaInP yellow-green light-emitting diodes with a tensile strain barrier cladding layer

A novel tensile strain barrier cladding (TSBC) structure is proposed which can effectively increase the potential barrier of the AlGaInP yellow-green light-emitting diodes (LEDs). It was found that the electroluminescence intensity of the multi-quantum well (MQW)+TSBC AlGaInP 573-nm LED is twice as large as that of the conventional MQW AlGaInP LED emitting at the same wavelength. It was also found that the MQW+TSBC AlGaInP LED is less heat sensitive than the MQW and MQW+multiquantum barrier (MQB) AlGaInP LEDs. These results indicate that the MQW+TSBC LED is useful particularly under high-temperature operation.

Investigation on the distribution of fluorine and boron in polycrystalline silicon/silicon systems

The behaviors of fluorine in BF~ implanted polycrystalline silicon (poly-Si) on silicon have been investigated in the annealing temperature range of 850 to 1100~ The distribution of fluorine atoms as functions of temperature and time have been monitored by the secondary ion mass spectroscopy (SIMS) and cross-sectional transmission electron microscopy (XTEM). The XTEM micregraphs revealed that fluorine bubbles are distributed in the poly-Si and at the original poly-Si/ Si interface after annealing. The locations of bubbles were found to correspond to the fluorine peaks in the SIMS depthconcentration profiles. The presence of the boron peak at the original poly-Si/Si interface is attributed to the gettering of boron atoms by the fluorine bubbles. Moreover, the boron profiles in the silicon substrates are sensitive to thermal budget due to the pileup of fluorine atoms at the pely-Si/Si interface. The pileup of fluorine at the poly-Si/Si interface leads to an enhancement of epitaxial regrewth of poly-Si films and the formation of fluorine bubbles. Consequently, higher surface dopant concentration and deeper junction depth were obtained.

Hydrogen Sensing Characteristics of a Pd/AlGaN/GaN Schottky Diode

In this paper, the interesting hydrogen sensing properties of a Pd-gate AlGaN/GaN Schottky diode are investigated. A significantly low detection limit of 850 ppb H2/air gas can be observed with increasing the temperature to 423 K. The experimental results indicate that hydrogen molecules cause great influences on the diode breakdown voltage. Also, the diode exhibits an ultrahigh sensing response of 2.04×105 at 423 K when exposure to a 9660 ppm H2/air gas. The transient response time and reversibility of the studied device can be improved by increasing the operating temperature.

Hydrogen sensing properties of a Pt-oxide-GaN Schottky diode

The interesting hydrogen sensing properties of a Pt-oxide-GaN metal-oxide-semiconductor-type Schottky diode are comprehensively studied and demonstrated. In the hydrogen-containing environment, the shift in current-voltage curves and decrease in turn-on voltage are found to be caused by the lowering of Schottky barrier height. Also, the corresponding series resistance is decreased from 191.8 (in air) to 155.3 Ω (for a 9970 ppm H2/air gas) at 30 °C. As the carrier gas is replaced by a nitrogen gas, a significant variation of 0.32 V and 19.56 Ω in the turn-on voltage Von and series resistance Rs values, respectively, is obtained at 30 °C, even at an extremely low hydrogen concentration of 4.3 ppm H2/N2. Since the oxygen atoms will be dissolved on the Pt metal surface and react with hydrogen atoms by the formation of hydroxyl and water, the number of adsorbed hydrogen atoms on the Pt surface is reduced. Moreover, the shorter response time constant and the larger initial rate of current density variation are ...

Further Suppression of Surface-Recombination of an InGaP/GaAs HBT by Conformal Passivation

Conformal passivation on an InGaP/GaAs HBT with significant reduction in the base surface-recombination effect is demonstrated. Not only dc behaviors but also RF performances are remarkably improved compared with the conventional emitter-ledge structure. Based on the conformal passivation, i.e., the base surface is covered by the depleted InGaP ledge structure and sulfur ((NH4)2Sx ) treatment, lower base surface-recombination current density, lower specific contact resistance, lower sheet resistance, higher current gain, higher collector current, and higher maximum oscillation frequency are obtained

Stress relaxation in the GaN∕AlN multilayers grown on a mesh-patterned Si(111) substrate

300×300μm2 crack-free GaN∕AlN multilayers of 2μm in thickness have been successfully grown on the Si(111) substrate patterned with the SixNy mesh by metal-organic chemical-vapor deposition. The in-plane stress exhibits a U-shape distribution across the “window” region, supported by the Raman shift of the GaN E2(TO) mode. This indicates a stress relaxation abruptly occurring near the edge of the window region due to the freestanding surface (11¯01) or (112¯2). The in-plane stress is almost relaxed at the corner of the window region due to three freestanding surfaces (11¯01), (112¯2), and (101¯1). The maximum in-plane stress is located near the surface of the multilayers at the center of the window region, supported by the Raman measurements and the failure observations. The role of the SixNy mesh in the stress relaxation is discussed.