Chang-Chin Yu

Study of GaN light-emitting diodes fabricated by laser lift-off technique

The fabrication process and performance characteristics of the laser lift-off (LLO) GaN light-emitting diodes (LEDs) were investigated. The LLO-GaN LEDs were fabricated by lifting off the GaN LED wafer structure grown on the original sapphire substrate by a KrF excimer laser at 248 nm wavelength with the laser fluence of 0.6 J/cm2 and transferring it onto a Cu substrate. The LLO-GaN LEDs on Cu show a nearly four-fold increase in the light output power over the regular LLO-LEDs on the sapphire substrate. High operation current up to 400 mA for the LLO-LEDs on Cu was also demonstrated. Based on the emission wavelength shift with the operating current data, the LLO-LEDs on Cu show an estimated improvement of heat dissipation capacities by nearly four times over the light-emitting devices on sapphire substrate. The LLO process should be applicable to other GaN-based LEDs in particular for those high light output power and high operation current devices.

Cathodoluminescent characteristics of ZnGa2O4 phosphor grown by radio frequency magnetron sputtering

Low voltage cathodoluminescent characteristics of ZnGa2O4 phosphor grown by rf magnetron sputtering have been investigated from 300 to 700 nm. The effects of substrate heating and annealing treatment on the luminescent characteristics are also studied. A blue cathodoluminescent emission peaked at 470 nm is observed. Better luminescent properties are achieved on the films which have crystal structure with a standard powder x‐ray diffraction pattern of ZnGa2O4. The effect of the strength of the ligand field on the resultant energy levels for the ZnGa2O4 phosphor is investigated. Low‐voltage phosphor films with excellent cathodoluminescent characteristics have been successfully developed in this research.

Comparison of p-Side Down and p-Side Up GaN Light-Emitting Diodes Fabricated by Laser Lift-Off

The performance characteristics of laser lift-off (LLO) freestanding InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) mounted on a Cu substrate with p-side up, and p-side down configurations were determined and compared. The InGaN/GaN MQW LED structures, which were originally fabricated on a sapphire substrate, were transferred to a Cu substrate by the LLO process into two different configurations, namely p-side up and p-side down with the same Ni/Pd/Au p-contact metallizations. Both p-side down and p-side up LLO-LEDs showed a higher current operation capability up to 400 mA than the original LEDs on the sapphire substrate. The p-side down LLO-LEDs showed a nearly eightfold increase in the light output power compared to the p-side up LLO-LEDs. The p-side down LLO-LEDs also showed a more stable wavelength emission spectrum than the p-side up ones.

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.

Enhanced Light Output in InGaN-Based Light-Emitting Diodes with Omnidirectional One-Dimensional Photonic Crystals

We have successfully designed and fabricated GaN-based light-emitting diodes (LEDs) containing an omnidirectional one-dimensional photonic crystal (1D PC). The 1D PC is composed of alternatively stacked TiO2 and SiO2 layers designed to possess a photonic band gap (PBG) within the blue regime of interest. With the same multiple quantum well (MQW) emission peak at approximately 450 nm and a driving current of 20 mA, the light output powers of the LED with and without the 1D PC are approximately 11.7 and 6.5 mW, respectively. The enhancement in light extraction of our LED with 1D PC demonstrates that a properly designed omnidirectional 1D PC can have a higher reflectance and a wider reflection angle than a conventional distributed Bragg reflector (DBR). Our work shows promising potential for the enhancement of output powers of commercial light emitting devices.

Improved Light Output Power of GaN-Based Light-Emitting Diodes Using Double Photonic Quasi-Crystal Patterns

The enhancement of light extraction from GaN-based light-emitting diodes (LEDs) with a double 12-fold photonic quasi-crystal (PQC) structure using nanoimprint lithography is presented. At a driving current of 20 mA on a transistor-outline-can package, the light output power of an LED with a nanohole patterned sapphire substrate (NHPSS) and an LED with a double PQC structure are enhanced by 34% and 61%, compared with the conventional LED. In addition, the higher output power of the LED with the double PQC structure is due to better reflectance on NHPSS and higher scattering effect on p-GaN surface using a 12-fold PQC structure pattern. These results provide promising potential to increase the output powers of commercial light-emitting devices.

Electrical and optical properties of beryllium-implanted Mg-doped GaN

We investigated the electrical and optical characteristics of beryllium implanted Mg-doped GaN materials. The Mg-doped GaN samples were grown by metalorganic chemical vapor deposition system and implanted with Be ions at two different energies of 50 and 150 keV and two different doses of about 1013 and 1014 cm−2. The implanted samples were subsequently rapidly thermal annealed at 900, 1000, and 1100 °C for various periods. The annealed samples showed an increase of hole concentration by three orders of magnitude from nonimplanted value of 5.5×1016 to 8.1×1019 cm−3 as obtained by Hall measurement. The high hole concentration samples also showed low specific resistance ohmic contact of about 10−3 Ω cm2 and 10−6 Ω cm2 using Ni/Au and Ni/Pd/Au metallization, respectively, without any further annealing process. It is also found from the temperature dependent photoluminescence that the activation energy of Mg dopants of the Be implanted samples has an estimated value of about 170 meV, which is nearly 30% lower ...

Investigation of beryllium implanted P-type GaN

Abstract We report the preliminary results of beryllium implanted into P-type GaN and the effects on the characteristic of Mg-doped P-GaN. These samples were implanted with Be ions were implanted with two different energies of 50 and 150 keV at two different doses of ∼10 13 and 10 14 cm −2 at room temperature. Surface morphology and photoluminescence measurements are presented.

Beryllium-Implanted P-Type GaN with High Carrier Concentration

We report the results of beryllium implantation in Mg-doped GaN to obtain a high hole concentration and lower activation energy for Mg. The metal organic chemical vapor deposition (MOCVD)-grown Mg-doped GaN samples were implanted with Be ions at two different energies of 50 keV and 150 keV and a dose of 1014 cm-2. The implemented samples were subsequently annealed at 1100°C for 60 s. The annealed samples showed an increase of hole concentration by three orders of magnitude from the non-implanted value of 5.5×1016 to 8.1×1019 cm-3 as determined by Hall measurement. The activation energy of Mg dopants for the implanted annealed samples estimated from the temperature dependence of the photoluminescence data is about 170 meV, which is nearly 30% lower than that for the as-grown samples.

Double Photonic Quasi-Crystal Structure Effect on GaN-Based Vertical-Injection Light-Emitting Diodes

GaN-based thin-film vertical-injection light-emitting diodes (VLEDs) with a double 12-fold photonic quasi-crystal (PQC) structure formed using nanoimprint lithography (NIL) and inductively coupled plasma reactive-ion etching (ICP-RIE) are fabricated and presented. At a driving current of 20 mA and with a chip size of 350 350 mm2, our thin-film LED with a double-12-fold-PQC structure gave a light output power of 40.5 mW, which is an increase of 77% when compared with the output power of a VLED without a PQC structure at a peak wavelength of 460 nm. In addition, the corresponding light radiation patterns show a narrow beam shape due to the strong guided light extraction on the n-GaN surface and the reflected light effect of the PQC structure formed in the vertical direction on the p-GaN surface. # 2010 The Japan Society of Applied Physics