Chia-Feng Lin

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.

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.

Growth and characterizations of GaN on SiC substrates with buffer layers

High quality GaN epitaxial layers were grown on 6H–SiC substrates by using low-pressure metalorganic chemical vapor deposition method. Samples employing a three-period GaN/Al0.08Ga0.92N (100u2009A/100u2009A) as a buffer layer produce a good quality GaN epitaxial layer, with mobility and carrier concentration of 612u2009cm2/V⋅s and 1.3×1017u2009cm−3 (at 300 K), respectively. The enhanced electron mobility in the Al0.08Ga0.92N/GaN heterostructures is also observed. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility at 4.2 K for the Al0.08Ga0.92N/GaN heterostructure are 5.8×1012u2009cm−2 and 5300u2009cm2/V⋅s, respectively. Strong SdH (Shubnikov–de Haas) oscillations were observed to confirm the two-dimensional electron gas (2DEG) phenomenon at the AlGaN/GaN top heterointerface. In addition, an extra SdH oscillation also resulted from the high-quality 2DEG channel of the GaN/AlGaN bottom heterointerface.

Etching damages on AlGaN, GaN and InGaN caused by hybrid inductively coupled plasma etch and photoenhanced chemical wet etch by Schottky contact characterizations

The surface damaging effects of the inductively coupled plasma (ICP) etch and the photoenhanced chemical (PEC) wet etch on AlGaN, GaN and InGaN were systematically investigated. The surface morphologies and the etch rates after ICP etch and PEC wet etch were explored to achieve optimum conditions for a hybrid etch technique. The etch rates increased with the ICP power or concentration of KOH(aq) and the surface roughness was less after PEC wet etch than it was after ICP etch. Schottky characterizations of GaN and AlGaN diodes after pure ICP etch, pure PEC etch and hybrid ICP/PEC etch were investigated to elucidate the damaging effects on the surfaces. It shows that the PEC etch is an effective way to achieve a damage-free surface for GaN-based materials, and the hybrid ICP/PEC etch can be used for applications which require high etch rate and damage-free surfaces.

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.

Chemical–Mechanical Lift-Off Process for InGaN Epitaxial Layers

An InGaN-based light-emitting diode (LED) structure was separated from a GaN/sapphire structure by inserting sacrificial Si-doped InGaN/GaN superlattice layers through a chemical–mechanical lift-off (CMLO) process. The CMLO process consisted of a band-gap-selective photoelectrochemical lateral wet etching process and a mechanical lift-off process. A lower elastic modulus and hardness of the lateral-etched LED structure were measured compared with the conventional LED structure, which indicated a weak mechanical property of the treated LED structure. The photoluminescence blue-shift phenomenon and the Raman redshift phenomenon indicated that the compressive strain from the bottom GaN/sapphire structure was released through the CMLO process.

InGaN Light-Emitting Diodes With the Inverted Cone-Shaped Pillar Structures

An InGaN-based light-emitting diode (LED) with an inverted cone-shaped pillar structure was fabricated through a plasma dry etching process and a photoelectrochemical (PEC) process. The undercut structure was fabricated through a bandgap-selective PEC etching process that occurred at the InGaN active layer. Then, the inverted cone-shaped pillar structure was formed through a bottom-up crystallographic etching process in a hot potassium hydroxide solution. The light-output power of the LED with an inverted cone-shaped pillar structure had a 42% enhancement compared with the standard LED without the pillar structure at a 20-mA operating current. A higher light intensity of the PEC-treated LED was observed around the mesa-edge region and the pillar structures as a result of a higher light-scattering process occurring at the inverted cone-shaped structure.

Simulation of InGaN Quantum Well Laser Performance Using Quaternary InAlGaN Alloy as Electronic Blocking Layer

Laser performance of an InGaN edge-emitting laser using a quaternary InAlGaN electronic blocking layer is investigated. Varying the aluminum (Al) composition in InAlGaN with a fixed indium (In) value (Al:In=5:1) indicates that a lower threshold current and higher characteristic temperature (T0) value can be obtained when the Al composition is higher than 20%. When Al=25%, the threshold current is reduced at the expense of a decreased T0 value from 149 to 130 K when the In composition increases from 1 to 7% in a temperature range of 300–370 K. The decreased T0 value is mainly attributed to the increase in electronic leakage current.

Fabrication and Micro-Photoluminescence Investigation of Mg-Doped Gallium Nitride Nanorods

High-density magnesium (Mg)-doped gallium nitride (GaN) nanorods were fabricated by inductively coupled plasma reactive ion etching from a GaN film and had a mean length of approximately 50 nm. A large blue-shift was observed in the photoluminescence (PL) peak energy of Mg-doped GaN nanorods under HeCd laser (325 nm) excitation. The PL spectra of the nanorods show a typical donor-acceptor-pair (DAP) emission at approximately 3.0 eV with a large blue-shift compared to that of the Mg-doped GaN film. The blue-shift energy increases from 8 meV to 67 meV as the excitation intensity varies from 12 kW/cm2 to 56 kW/cm2. Possible reasons for the power dependence of the spectral shift in the PL emission energy are discussed.