Yi-Lun Chou

Nanometer-scale conversion of Si3N4 to SiOx

It has been found that atomic force microscope (AFM) induced local oxidation is an effective way for converting thin (<5 nm) Si3N4 films to SiOx. The threshold voltage for the 4.2 nm film is as low as 5 V and the initial growth rate is on the order of 103 nm/s at 10 V. Micro-Auger analysis of the selectively oxidized region revealed the formation of SiOx. Due to the large chemical selectivity in various etchants and great thermal oxidation rate difference between Si3N4, SiO2, and Si, AFM patterning of Si3N4 films can be a promising method for fabricating nanoscale structures.

Enhanced characteristics of blue InGaN/GaN light-emitting diodes by using selective activation to modulate the lateral current spreading length

We have studied the characteristics of blue InGaN∕GaN multiquantum-well light-emitting diodes (LEDs) after reducing the length of the lateral current path through the transparent layer through formation of a peripheral high-resistance current-blocking region in the Mg-doped GaN layer. To study the mechanism of selective activation in the Mg-doped GaN layer, we deposited titanium (Ti), gold (Au), Ti∕Au, silver, and copper individually onto the Mg-doped GaN layer and investigated their effects on the hole concentration in the p-GaN layer. The Mg-doped GaN layer capped with Ti effectively depressed the hole concentration in the p-GaN layer by over one order of magnitude relative to that of the as-grown layer. This may suggest that high resistive regions are formed by diffusion of Ti and depth of high resistive region from the p-GaN surface depends on the capped Ti film thickness. Selective activation of the Mg-doped GaN layer could be used to modulate the length of the lateral current path. Furthermore, the ...

Improvement of Surface Emission for GaN-Based Light-Emitting Diodes With a Metal-Via-Hole Structure Embedded in a Reflector

This work demonstrates the feasibility of gallium nitride (GaN)-based light-emitting diodes (LEDs) with a metal-via-hole structure embedded in a reflector on the backside of sapphire substrate. Luminescence intensity for the surface-emitting LEDs is enhanced by mirroring efficaciously the downward light emitted from the InGaN/GaN multiquantum wells (MQWs) owing to the deep ladder-shaped inclined reflector on the backside of substrate. A metal-via-hole structure with a deep ladder shape was also processed with a wet-etching method at a high temperature to pattern the sapphire substrate deeply. The electroplating method was used to fill the area of patterned sapphire substrate with copper subsequently to produce a heat spreader path through the metal-via-hole and to strengthen the sapphire substrate with the void structure. Experimental results indicate that the GaN-based surface-emitting LEDs with a reflector for the planar, deep ladder-shaped sapphire, and metal-via-hole structures exhibit a luminescence intensity of 37%, 178%, and 226%, respectively, which are higher than the conventional ones under an injection current of 20 mA. A more stable peak wavelength shift is also observed for the metal-via-hole LED structure.

Thermal Stability for Reflectance and Specific Contact Resistance of Ni/Ag-Based Contacts on p-Type GaN

In this study, we investigated the thermal stability of Ni/Ag-based alloy contacts on p-type GaN. We observed the morphology of aggregated Ag for the Ni/Ag bilayer on p-type GaN after annealing it at 500°C in an O 2 ambient. To improve the thermal stability, we deposited a Ni/Ag/Au trilayer onto p-type GaN. In this case, Ag aggregation was retarded after thermal annealing, and the specific contact resistance exhibited improved stability. Furthermore, because strong interdiffusion of the Au and Ag layers leads to poor reflectance, we added a diffusion barrier layer into the system; i.e., we deposited Ni/Ag/Ti/Au onto p-type GaN, with the Ti layer playing the role of the diffusion barrier. After annealing, the contact exhibited diminished Ag aggregation and a lower level of interdiffusion of the Au and Ag layers. We investigated the effect of the annealing time (at 500°C in an O 2 ambient) on the properties of the Ni/Ag (1/150 nm), Ni/Ag/Au (1/150/150 nm), and Ni/Ag/Ti/Au (1/150/500/150 nm) layers, namely, their values of specific contact resistance, determined using a modified transmission line model, and reflectance at 465 nm. According to analyses by using scanning electron microscopy and secondary-ion mass spectrometry, we determined that the aggregation of Ag and the interdiffusion of Au and Ag within the Ni/Ag/Ti/Au construct were both minimized with the presence of the Ti layer, thereby improving the thermal stability of the contact on the p-type GaN.

1.3 μm Strain-Compensated InGaAsP Planar Buried Heterostructure Laser Diodes with a TO-Can Package for Optical Fiber Communications

In this paper, we report on the realization of the 1.3 μm strain-compensated InGaAsP buried heterostructure (BH) laser diodes (LDs) by using an Fe-doped semi-insulating InP layer. The performances of LDs are characterized by light output power, internal quantum efficiency, modal gain, characteristic temperature (To), and dynamic response. As a result of the good confinement of the injection carriers within the strained-compensated multiple quantum well (SC-MQW) and the better heat sink for thermal dissipation, the BH LDs exhibit a threshold current of 9 mA, a slope efficiency of 0.296 mW/mA, and a maximum light output power of 11.8 mW/facet at 76 mA. Besides, the transparent current density and modal gain are estimated as 106 A/cm 2 and 12.5 cm -1 , respectively, for the fabricated LDs. Otherwise, the BH LD with a facet coating is beneficial to get a lower threshold current, a higher light output power, and an improved T 0 value as compared to the conventional ridge-waveguide LD. Furthermore, this transistor outlook (TO)-packaged BH LD for small-signal analyses does not show any parasitic effects at low frequencies and has a maximum modulation frequency of 9.6 GHz at 80 mA. Finally, the BH LD also exhibits the promising potential for high speed data transmission as evaluated from the 10 Gb/s eye-opening feature. These results imply that the 1.3 μm TO-packaged SC-MQW InGaAsP LDs are excellent candidates for use in high speed optical fiber communications.

Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation of the Mg-Doped GaN Layer

In this study, we used the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN-GaN multiple quantum-well blue light-emitting diodes (LEDs). To access the current blocking region after forming a periphery high-resistance ring-region of the Mg-doped GaN layer and to reduce the degree of carrier trapping by the surface recombination centers, we deposited a titanium film onto the Mg-doped GaN epitaxial layer to form a high-resistance current blocking region. To characterize their luminescence performance, we prepared LEDs incorporating titanium films of various widths of the highly resistive current blocking layer. The hole concentration in the Mg-doped GaN epitaxial layer decreased from 3.45times1017 cm-3 to 3.31times1016cm-3 after capping with a 250-nm-thick layer of titanium and annealing at 700 degC under a nitrogen atmosphere for 30 min. Furthermore, the luminescence characteristics could be improved by varying the width of the highly resistive region of the current blocking area; in our best result, the relative electroluminescence intensity was 30% (20 mA) and 50% (100 mA) higher than that of the as-grown blue LEDs

Using the Taguchi method to improve the brightness of AlGaInP MQW LED by wet oxidation

To increase the external quantum efficiency of a light-emitting diode (LED) while limiting its forward voltage (Vf), we prepared both (AlxGa1-x)0.5In0.5 P LED and buried oxides through selective wet oxidation of the AlAs layers of AlAs-GaAs distributed Bragg reflectors. The wet oxidation process forms a stable AlxO material that acts as an insulation layer and affects both the carrier and optical confinements. To determine the tradeoff conditions for LED oxidation, we used the Taguchi method which is a robust technique that is often used to analyze significant trends that occur under a set of oxidation conditions. In this study, we used an L9 orthogonal array to measure the effects that a series of factors have upon the maximum brightness performance of the LED in an effort to limit the values of Vf. Relative to the as-grown LED, the oxidized LED that had been treated under the tradeoff wet-oxidation conditions displayed a sharply enhanced brightness (62.4% increase) in conjunction with only a slightly increased value of Vf (only a 24.5% increase)

Effect of current spreading on luminescence improvement in selectively oxidized AlGaInP light-emitting diodes

We have systematically investigated the luminescence performance of AlGaInP light-emitting diodes with a wet-oxidized AlAs/AlGaAs distributed Bragg reflector structure. The peak intensity for a 20-μm-radius aperture is 4.2 times greater than that for a 100-μm-radius aperture at the same current density. We believe that this significant improvement in the emission characteristics with the Al-based native oxides is the result of the current-spreading effect. With an increase in the lateral depth of the oxidized layer, the near-field patterns become more centralized. This is attributed to the confinement of the current to the region about the unoxidized aperture. We also found that the centralized carriers contribute to the enhancement of the external quantum efficiency per unit area up to factor of 58.6.

SiOx-planarized and transistor outlook-packaged oxide-confined vertical-cavity surface-emitting lasers with ring-shape geometry for high-speed (10Gb∕s) operation

This study explores an alternative approach to fabricate the 850nm SiOx-planarized ring-shape vertical-cavity surface-emitting lasers (VCSELs) with stable lasing emission and high-speed operation. The fabricated ring-shape VCSELs have an inner oxide diameter of 9μm and an outer oxide diameter of 15μm. These devices show a threshold current of 3.65mA, a maximum light output power of 7.5mW at 25mA, and a differential resistance of 65Ω at room temperature. Furthermore, they exhibit a stable dual-mode behavior over the operation current of 25mA. Moreover, this TO-packaged 850nm VCSEL for small-signal analyses shows a maximum modulation frequency of about 8GHz, which is corresponding to a modulation-current efficiency factor of 2.47GHz∕mA1∕2. This VCSEL also shows a clear and symmetric eye-opening feature at 10.3Gbytes∕s and 18mA for back-to-back and 66m transmission tests. Based on these results, the excellent high-speed performance can be fulfilled by the SiOx-planarized and TO-packaged oxide-confined VCSELs...

Improved Mode Stability of Long-Wavelength GaInAsN Vertical-Cavity Surface-Emitting Lasers by Using the Selective Ge-Coating Layer

In this article, we demonstrate the fabrication and characteristics of the GaInAsN vertical-cavity surface-emitting lasers (VCSELs) with a stable and fundamental transverse mode by using the Ge antireflection (AR) coating, selective wet oxidation, and thick silicon oxide planarization processing techniques. The Ge―semiconductor interface provides a lower reflectivity, as compared with the semiconductor―air interface, and thus can be used as an AR coating to purify the lasing spectra for the oxide-confined VCSELs. The VCSELs with a 13 μm oxide aperture along with a 7 μm diameter Ge coating exhibit a maximum light output power of 0.63 mW and a single transverse mode with a side-mode suppression ratio over 30 dB at an injection current up to 15 mA. In addition, the SiO x -planarized GaInAsN VCSELs show a clear and symmetric eye diagram operated at 1.25 Gb/s at 11.6 mA. These results show that such single-mode VCSELs are excellent candidates for use in high speed long-distance data communications.