Byungjin Ma

High-power 660-nm GaInP-AlGaInP laser diodes with low vertical beam divergence angles

We present 660-nm GaInP-AlGaInP ridge multiple-quantum-well laser diodes (LDs) reliably operating at high output power over 200 mW at 70/spl deg/C not showing unstable higher order lateral modes owing to an adoption of a dry etching method instead of a conventional chemical wet etching for realizing steep ridge sidewalls. Employing an optimized two-step n-cladding layer LDs produced very narrow horizontal and vertical beam divergence angles of 8.6/spl deg/ and 15.3/spl deg/, respectively. To the best of our knowledge, this vertical beam divergence angle is the lowest value ever reported in high-power 660-nm LDs operating over 200 mW and is expected to play an important role in minimizing the coupling loss between LD and passive optical components in digital versatile disc system.

Thermal properties and lifetime comparison of various ceramic-package light-emitting diodes

We developed two thermal structures with heat-conducting paths at the bottom side of low temperature cofired ceramic (LTCC) LED packages and compared their thermal properties and reliability. Using a computational fluidic dynamics (CFD) simulation, we were able to anticipate the improvement of the thermal properties. In addition, we experimentally confirmed the expected thermal properties using a method based on thermal transient responses. The measured thermal resistances of the small-via structure and the thermalslug structure were about 50% and 40% of the value of a normal structure with no via or slug, respectively. Lifetime tests of each LED package over 4,000 hours were carried out to compare the lifetimes of the LTCC LED packages and to investigate the relationships between the thermal properties and the lifetimes of the LTCC LED packages. We were able to estimate a lifetime of over 32,000 hours for the LTCC LED packages with thermal paths while the lifetime of the normal LTCC LED packages was estimated to be about 18,200 hours. Also, we verified the strong relationship between the thermal property and the lifetime in LTCC LED packages.

Proposal of New Measurement Method for Internal Quantum Efficiency of Light-Emitting Diodes

A new evaluation method for effective internal optical power (IOP) and internal quantum efficiency (IQE) of light-emitting diodes (LEDs) is demonstrated. This method is based on the optical and thermal properties of LEDs. By using this proposed method, the effective IOP and the IQE of LEDs could be directly extracted from the measurements of external optical power (EOP) and junction temperature of LEDs. This method needs no assumption of the injection efficiency of carriers in the LEDs and no measurement-condition limitation of low current-injection level.

Realization of High-Power Highly Efficient GaInP/AlGaInP Ridge Laser Diodes for Recordable/Rewritable Digital Versatile Discs

Very efficient and reliable GaInP/AlGaInP red multiple-quantum-well laser diodes for recordable/rewritable digital versatile disc (DVD-R/RW) were successfully developed by low-damage fabrication. Because we adopted dry etching instead of conventional chemical wet etching for the steep ridge sidewalls of laser diodes (LDs), devices could be operated at a high output power over 200 mW at 70°C without showing any unstable higher-order transverse modes. In-situ monitoring tools in metal organic chemical vapor deposition (MOCVD) and dry etching were useful for controlling process parameters precisely, which enhances device characteristics and maintains the reproducibility of device fabrication. A very small beam aspect ratio of 1.78 with a very low vertical beam divergence angle of 15.3° at a high output power was achieved by optimizing the two-step n-cladding layer in our LD structures. These low values of the vertical beam divergence angle and aspect ratio are essential for minimizing the coupling loss between the LD and the objective lens in DVD pickups.

Multi-channel and real-time monitoring system for junction temperature of light-emitting diodes

In order to enhance the accuracy of lifetime estimation in the LED packages, we developed a multi-channel real-time monitoring system for LED characteristics. This characterizing system includes a precise junction-temperature measurement of the LED packages. A 2, 500-hour lifetime test had been carried out and the junction temperatures of the LED packages had been monitored. Our proposing method could consider a deviation of the thermal property in the LED packages and we could filter out thermally-abnormal samples in a lifetime-estimation procedure.

Power cycling test of power semiconductor based on junction temperature monitoring

Due to the TCE(thermal coefficient of expansion) mismatch, power semiconductor suffers from bonding crack and delamination due to large temperature swing. For verification of power semiconductor reliability, power cycling test is conducted. Junction temperature evaluation is most important factor which determines validity of the acceleration life test. In this paper, conceptual methodology how to evaluate the real-time junction temperature during the power cycling acceleration test was introduced. Junction temperature is evaluated by forward voltage drop measurement, and the junction temperature is real-time monitored during the power cycling test. Proposed real-time junction temperature monitoring technology was effective for the power cycling test control, and it is expected to be very effective for various applications.

Formation of vertical ridge structure in 660nm laser diodes for high power single mode operation

We report a method for formation of a ridge with almost vertical sidewalls in the fabrication of ridge stripe AlGaInP laser diode structures, which cannot be easily achieved when a conventional wet-etch process is used, for high power operation with a fundamental transverse mode. By depositing additional oxides protecting layers on the sidewalls after the dry-etch process, a wet-etch process, which is necessary to remove the plasma-induced damaged portion of the etched region, can be safely applied and the vertical ridge shape maintained. Using this method, the ideally rectangular ridges were obtained and the single mode operation of 660nm laser diode with up to 270mW output power was successfully achieved.

Investigation on solder voids in flip-chip light-emitting diodes using thermal transient response

In order to reduce the cost of the LED packages and to minimize the thermal budget, the chip-on-board (COB) packages and the chip-scale package (CSP) LEDs, based on flip-chip die bonding process, have been developed. Although the flip-chip die bonding process for the COB and the CSP LEDs is useful to decrease the thermal resistance, a precise process control for minimizing voids in solder joint and an in-line inspection for bonding quality check are needed for reliability. We proposed a simple in-line void inspection method based on the thermal transient response of LED junction temperature. In order to carry out the feasibility test, we made three LED package groups showing different solder void qualities. By measuring voltages at two points in time domain, we could distinguish the LED packages showing a poor solder void quality.

In-situ monitoring on junction temperature for degradation analysis of light-emitting diodes

A precise monitoring on the junction temperature of the light-emitting diode (LED) packages during a lifetime tests is very helpful to make a diagnosis of their quality change and to analyse the optical degradation. We proposed a new model for internal quantum efficiency (IQE) of the LED packages as a function of the junction temperature and developed an in-situ IQE measurement system for an investigation of the optical degradation mechanism in the LED packages. A lifetime test for 2,500 hours had been carried out with in-situ monitoring on the relative optical power and the sub-threshold leakage current. In addition, we had measured the junction temperature and the IQE as a tool for the degradation analysis of the LED packages. No change in the IQE and the leakage current throughout the lifetime test has been observed although there was a 10% optical degradation of the LED package.

Proposal of new thermal resistance for light-emitting diodes

In order to estimate exact thermal resistance of light-emitting diodes (LED), we introduced a new concept of effective internal optical power (IOP) of LED packages. It is because all photons generated at active region of LED chip cannot emit outside due to the difference of refractive index between a LED chip and an encapsulant material. The meaning of the effective IOP of LED package is non-thermal power of total power in LEDs. We derived a model for effective IOP and proposed a new thermal resistance using the effective IOP. To investigate pure thermal effect and distinguish optical effect in the LED packages, we carried out a computational fluidic dynamics simulations.