Tao-Hung Hsueh

Gallium Nitride Nanorods Fabricated by Inductively Coupled Plasma Reactive Ion Etching

We report a novel method of fabricating gallium nitride (GaN) nanorods of controllable dimension and density from GaN epitaxial film using inductively coupled plasma reactive ion etching (ICP-RIE). The GaN epitaxial film was grown on a sapphire substrate by metal-organic chemical vapor deposition. Under the fixed Cl2/Ar flow rate of 10/25 sccm and ICP/bias power of 200/200 W, the GaN nanorods and array were fabricated with a density of 108–1010 cm-2 and dimension of 20–100 nm by varying the chamber pressure from 10 to 30 mTorr. The technique offers one-step, controllable method for the fabrication of GaN nanostructures and should be applicable for the fabrication of GaN-based nano-optoelectronic devices.

Enhancement in light output of InGaN-based microhole array light-emitting diodes

InGaN-based microhole array light-emitting diodes (LEDs) with hole diameters (d) of 3-15 /spl mu/m were fabricated using self-aligned etching. The effects of size on the device characteristics, including current density-voltage and light output-current density, were measured and compared with those of conventional broad-area (BA) LEDs fabricated from the same wafer. The electrical characteristics of the devices are similar to those of conventional BA LEDs. The light output from the microhole array LEDs increases with d up to 7 /spl mu/m. However, the light output declined as d increased further, perhaps because of the combination of the enhancement in extraction efficiency caused by the large surface areas provided by the sidewalls and the decrease in area of light generation by holes in the microhole array LEDs. The ray tracing method was used with a two-dimensional model in TracePro software. The findings indicate that an optimal design can improve the light output efficiently of the microhole array LEDs.

Characterization of InGaN/GaN Multiple Quantum Well Nanorods Fabricated by Plasma Etching with Self-Assembled Nickel Metal Nanomasks

High-density (3.0×1010 cm-2) InGaN/GaN multiple quantum well (MQW) nanorods were fabricated from an as-grown bulk light-emitting diode structure by inductively coupled plasma dry etching with self-assembled nickel metal nanomasks. The self-assembled nickel metal nanomasks were formed by rapid thermal annealing of a nickel metal film at 850°C for 1 min. The influence of the thicknesses of the Ni metal film on the dimensions and density of the nanorods was also investigated. The structural and optical properties of the InGaN/GaN MQW nanorods were established using field emission scanning electron microscopy, transmission electron microscopy and photoluminescence measurements. The diameters and heights of nanorods were estimated to be 60 to 100 nm and more than 0.28 µm, respectively. The peak emission wavelength of the nanorods showed a blue shift of 5.1 nm from that of the as-grown bulk. An enhancement by a factor of 5 in photoluminescence intensity of the nanorods compared with that of the as-grown bulk was observed. The blue shift is attributed to strain relaxation in the wells after dry etching, the quantum confinement effect, or a combination of the two, which results in the enhancement of emission intensity.

Fabrication and photoluminescence of InGaN-based nanorods fabricated by plasma etching with nanoscale nickel metal islands

InGaN-based nanorods with a rod density of ∼3.0×1010cm−2 were fabricated from a light-emitting diode structure by an inductively coupled plasma dry-etching with nanoscale nickel metal islands. The nanoscale nickel metal islands were formed from a Ni film by a rapid thermal annealing at 850°C for 1min. The influence of thicknesses of Ni metal film on the diameter and density of nanorods was also investigated. Structural and optical properties of the InGaN-based nanorods were studied with field-emission scanning electron microscopy, transmission electron microscopy, and photoluminescence. The diameters and heights of nanorods were estimated to be 60–100nm and more than 0.28μm, respectively. The emission-peak wavelength of nanorods showed a blueshift of 5.1nm from that of the bulk structure. An enhancement by a factor of five times in photoluminescence intensity of nanorods compared to that of the bulk structure was also observed in this work. The blueshift is attributed to the strain relaxation in the well,...

Effects of In and Ga interdiffusion on the optical gain of InGaN/GaN quantum well

In this study, we analyze the effects of thermal annealing by calculating the optical gain in the InGaN/GaN quantum well. The interdiffusion of Ga and In atoms across the interface of the well and the barrier resulting from thermal treatments is described by Fick’s law. The strong piezoelectric effect due to lattice mismatch in the InGaN/GaN quantum well is also considered in the calculation. The results confirm that the thermal annealing can induce an increase of the optical gain. However, an excessive annealing might result in decreasing the optical gain in the InGaN/GaN quantum well. The maximum optical gain can be obtained at a diffusion length of 4A of In and Ga atoms. There is a good agreement between the experimental data in literature and the optimized diffusion length studied in this work.

10 Gb/s single-mode vertical-cavity surface-emitting laser with large aperture and oxygen implantation

High-speed single transverse mode 850 nm vertical cavity surface emitting lasers (VCSELs) with large emission aperture with a diameter of 8 µm were fabricated. These VCSELs exhibit good performance with threshold currents of 1.5 mA, a single transverse mode emission within the full operational range and a maximum output power of 3.8 mW. The large aperture is advantageous to these VCSELs with a smaller dynamic resistance (60 Ω) than that of conventional single-mode VCSEL. These single-mode VCSELs also demonstrate superior high-speed performance up to 10 Gb s−1.

Performance of 850 nm AlGaAs/GaAs implanted VCSELs utilizing silicon implantation induced disordering

Abstract In this paper, we report a novel implanted vertical surface emitting lasers (VCSELs) utilizing silicon implantation induced disordering. The VCSELs exhibit kink-free current–light output performance with threshold currents ∼2.4 mA, and the slope efficiencies ∼0.45 W/A. The threshold current change with temperature is minimal and the slope efficiency drops less than ∼30% when the substrate temperature is raised to 90 °C. The eye diagram of VCSEL operating at 2.125 Gb/s with 7 mA bias and 10 dB extinction ratio shows very clean eye with jitter less than 30 ps. We have accumulated life test data up to 5000 h at 100 °C/20 mA with exceptional reliability and the WHTOL (high temperature and high humidity 85 °C/85 operating lifetime) biased at 8 mA has passed over 2000 h.

Photoluminescence from In0.3Ga0.7N/GaN multiple-quantum-well nanorods

The fabrication of In0.3Ga0.7N/Ga Nm ultiple-quantum-well nanorods with diameters of 60–100 nm and their optical characteristics performed by micro-photoluminescence measurements are presented. The nanorods were fabricated by inductively coupled plasma dry etching from a light-emitting diode wafer. The structure and surface properties of fabricated nanorods were verified by the field emission scanning electron microscopy and the transmission electron microscopy. The photoluminescence (PL) spectra with sharp linewidths of typically 1.5 nm were observed at 4 K. The excitation-power-dependent spectra show that no energy shift was observed for these sharp peaks. Moreover, increasing the excitation power instead leads to an occurrence of new, sharp PL peaks at the higher energy tail of the PL spectra, which suggest that excitons are strongly confined in quantum-dot-like regions or localization centres.

Thermal annealing effects on the optical gain of InGaN/GaN quantum well structures

Abstract In this work, the variation of the optical gain in the InGaN/GaN quantum well after thermal annealing is simulated. The potential profile change of the quantum well resulting from the interdiffusion of Ga and In atoms across the interface of the well and the barrier during the thermal treatments is assumed to follow Fick’s law. The results show that the thermal annealing can induce an increase of the optical gain in the InGaN/GaN quantum well. The maximum optical gain can be obtained at a diffusion length of 0.4 nm of In and Ga atoms. However, an excessive annealing may result in decreasing the optical gain. There is a good agreement between the experimental data in literature and the optimized diffusion length studied in this work.

GZO/GaN Schottky Barrier Ultraviolet Band-pass Photodetector with a Low-temperature-grown GaN Cap Layer

Aluminum gallium nitride (AlGaN) alloys are most promising materials for the fabrication of high-sensitivity solar/visible-blind photodetectors (PDs), since it has a wide direct band gap (3.4~6.2 eV at room temperature) and a high saturation electron drift velocity. In the past years, various types of GaN-based photodetectors have been demonstrated, such as p-n junction photodiodes, p-i-n PDs, p-π-n PDs, Schottky barrier PDs, andmetal-semiconductormetal (MSM) PDs [1-5]. The most important key point to fabricate Schottky barrier PDs with high responsivity and low leakage current is the performance of Schottky contacts (SCs). It has been reported that various metals and transparent conducting oxide films deposited on GaN could achieve high performance SCs. In addition to the choice of contact metals, leakage current of SCs also depends strongly on the threading dislocation (TD) density of the GaN layers. In this study, we grew a LTG GaN cap layer on conventional high-temperature-grown GaN layer associated with the use of GZO films deposited on the LTG GaN to form the SB PDs. In this work, the samples used in this work were grown on c-face sapphire (0001) substrates by metal organic chemical vapor deposition. As shown in Fig. 1, a 30-nm-thick GaN nucleation layer was grown first at 550 C, and followed by a 3-μm-thick Si-doped n-GaN and a 1-μm-thick un-doped GaN layer grown at 1050C. Finally, the wafers were capped with a 30-nm-thick un-doped GaN layer grown at temperature of 550C. It should be noted that the LTG GaN cap layer behaves in a kind of insulator with a sheet resistivity larger than 10 Ω/□. To fabricate the GaN SB PDs, Cl2-based plasma dry etching was applied to expose the n-GaN underlying layer. A GZO film with a thickness of about 200 nm was deposited on top of the samples by DC magnetron sputtering. The GZO target used in the DC magnetron sputtering containes 97% ZnO and 3% Ga2O3, in terms of weight percentage. The deposited GZO thin films exhibited a typical transmittance of over 80% at an incident wavelength of 360 nm, which implies a potential alternative to replace the conventional transparent contact layers, such as Ni/Au bilayer metal, which are used to serve as a transparent Schottky and Ohmic contact on GaN-based PDs and LEDs [6], respectively. Samples were then annealed at 700°C for 1 min in N2 ambient by rapid thermal annealing in order to reduce resistivity of the GZO films. The typical resistivity of the GZO films after annealing was measured to be around 5×10 Ω-cm determined by Hall-effect measurement [7]. Then, a Cr/Au (50/200 nm) bilayer was deposited on the exposed n-GaN and the GZO served by e-beam evaporator to serve as n-type Ohmic contacts and anode electrodes, respectively. The diameter of the circular devices fabricated in this work was maintained at 500 μm. Here, PD-I and PD-II are corresponding to the sample with and without the LTG GaN cap layer, respectively. The current-voltage (I-V) characteristics were measured at room temperature using HP4156C semiconductor parameter analyzer. Spectral responsivity of these SB PDs was measured using a Xe arc lamp and a calibrated monochromator as the light source. The monochromatic light was calibrated by Si photodiode and then illuminated onto the front side of SB PDs.