Chen-Fu Chu

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.

Low-resistance ohmic contacts on p-type GaN using Ni/Pd/Au metallization

In this letter, a low-resistance ohmic contact on p-type GaN using an alloy of Ni/Pd/Au is reported. The Mg doped p-type GaN samples were grown by metalorganic chemical vapor deposition with a carrier concentration of 4.1×1017 cm−3. The as-grown Mg doped samples were deposited with Ni (20 nm)/Pd (20 nm)/Au (100 nm) and then annealed in air, nitrogen, and oxygen ambient conditions at different annealing temperatures ranging from 350 to 650 °C. Linear I–V ohmic characteristics were observed with specific resistance as low as 1.0×10−4 Ω cm2 for the samples annealed in oxygen atmosphere. Similar contact metal composition was also deposited on Be-implanted p-type GaN samples with a carrier density of 8.1×1019 cm−3. Without further annealing process, the samples show good ohmic contact with a lowest specific resistance of 4.5×10−6 Ω cm2.

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.

Fabrication of Large-Area GaN-Based Light-Emitting Diodes on Cu Substrate

A large-area GaN-based light-emitting diode (LED) 1000×1000 µm2 in size with a p-side down configuration was fabricated using wafer bonding and laser lift-off (LLO) techniques. The thin GaN LED was transferred onto a copper substrate without peeling or cracks. The large-area LEDs showed a uniform light-emission pattern over entire defined mesa area without a transparent contact layer on the p-type GaN. The operating current of the large-area LEDs can be driven up to 1000 mA with continuously increasing light output-power. The light output-power is 240 mW with a driving current of 1000 mA. Large-area emission and high current operation make the LLO-LEDs applicable to high-power LED applications.

Effects of different n-electrode patterns on optical characteristics of large-area p-side-down InGaN light-emitting diodes fabricated by laser lift-off

Large-area p-side-down InGaN light-emitting diodes (LEDs) 1000×1000 µm2 in size have been fabricated by laser lift-off (LLO). The p-side-down LEDs with different geometric patterns of n-electrodes were fabricated to investigate electrode pattern-dependent optical characteristics. The current crowding effect was observed in the large-area p-side-down InGaN LLO-LEDs. A LED with a well-designed n-electrode pattern shows a uniform distribution of light emission and a higher output power due to uniform current spreading. The output power saturation induced by the current crowding effect was investigated. In the absence of a transparent contact layer for current spreading, the n-electrode pattern has a marked influence on the current distribution and the consequent light output power of the large-area p-side-down LEDs.

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.

High etching rate of GaN films by KrF excimer laser

Abstract A study of laser processing of gallium nitride (GaN) material is reported. A pulsed KrF excimer laser at 248 nm with 20-nsec pulse width and 1 Hz repetition rate is used to etch the GaN film. We establish the material etching parameters under different environmental conditions. By changing the pulsed energy at constant pulse numbers, ablation of GaN surface was observed at threshold laser fluence about 0.3 J cm −2 . Laser etching increase with reducing environment pressure. At 1.0 J cm −2 laser fluence, the etching rate is about 35 nm per pulse at atmosphere pressure and increases to 60 nm per pulse at low pressure. The etched depth also increases with increasing laser fluence. The surface morphology of the etched surface was also investigated.

Formation of excellent shallow n + p junctions by As + implantation into thin CoSi films on Si substrate

Abstract Excellent shallow n + p junctions with a leakage current density of about 0.6 nAcm 2 have been fabricated by As + implantation into thin CoSi films on Si substrates and subsequent drive-in/silicidation. The resultant junction characteristics are strongly affected by the implant conditions. A deeper as-implanted dopant profile is more helpful to drive the dopant within the silicides into the junction regions. Because the silicidation facilitates damage annihilation, the implant conditions forming excellent junctions in this scheme own large process window and thus yield low thermal budgets. Rapid thermal annealing was not favorable to forming junctions in this scheme because of its short annealing times for the arsenic drive-in.

Effect of different n-electrode patterns on optical characteristics of large-area p-side down InGaN light-emitting diodes fabricated by laser lift-off

Large-area (1000×1000 μm2) p-side down InGaN light-emitting diodes (LEDs) have been fabricated by laser lift-off (LLO) technique. The p-side down LEDs with different geometric patterns of n-electrode were fabricated to investigate electrode pattern-dependent optical characteristics. Current crowding effect was first observed in in the p-side down InGaN LLO-LEDs. The LEDs with well designed n-electrode shows a uniform distribution of light-emitting pattern and higher out put power due to uniform current spreading and minimization of thermal effect. The output power saturation induced by current crowding in the LEDs with simplest geometric n-electrode was demonstrated. In absent of transparent contact layer for current spreading, the n-electrode pattern has remarkable influence on the current distribution and consequently the light output power of the large-area p-side down LEDs.