Haeng-Keun Ahn

Structural and optical properties of ZnO∕Mg0.1Zn0.9O multiple quantum wells grown on ZnO substrates

ZnO∕Mg0.1Zn0.9O multiquantum-well (MQW) structures were grown on ZnO substrates by molecular beam epitaxy. Abrupt interfaces and well/barrier width in the MQWs were confirmed by x-ray diffraction measurement and transmission electron microscopy. The transition energy of the localized exciton in the ZnO∕Mg0.1Zn0.9O MQWs with well/barrier width of 5∕8nm was found to be about 3.375eV at low temperature, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 19.2 and 5.4meV, respectively. The transition energy showed no shift with excitation power, indicating that the polarization-induced electric field is negligible in the ZnO∕Mg0.1Zn0.9O MQW structures.

Metal-organic chemical vapor deposition growth of InGaN/GaN high power green light emitting diode: Effects of InGaN well protection and electron reservoir layer

We investigated the effects of the well protection layer (WPL) and electron reservoir layer (ERL) on the emission properties of InGaN/GaN green multiple quantum wells (MQWs). In order to increase their emission wavelength by preventing the volatile InGaN well, a thin GaN WPL was coated subsequently on each well layer at the same temperature before ramping-up the temperature to grow the GaN barrier. It was found that the WPL directly influenced the indium content and optical properties of the MQW. The indium content was in fact increased, as was evident from the x-ray diffraction and photoluminescence experiments. Then, to explore the possibility of enhancing the quantum efficiency by increasing the electron capture rate, a superlattice ERL composed of ten pairs of InGaN/GaN was embedded between the MQW and n-GaN. The electroluminescence intensity of the green light emitting diode with the ERL was up to three times higher than that of the diode without the ERL. These results imply that the carrier capture ...

Comparative study of electroabsorption in InGaN∕GaN quantum zigzag heterostructures with polarization-induced electric fields

We present a comparative study on InGaN∕GaN quantum zigzag structures embedded in p-i-n diode architecture that exhibit blue-shifting electroabsorption in the blue when an electric field is externally applied to compensate for the polarization-induced electric field across the wells. With the polarization breaking their symmetry, the same InGaN∕GaN quantum structures redshift their absorption edge when the external field is applied in the same direction as the well polarization. Both computationally and experimentally, we investigate the effects of polarization on electroabsorption by varying compositional content and structural parameters and demonstrate that electroabsorption grows stronger with weaker polarization in these multiple quantum well modulators.

In-situ Observation on Micro-Fractural Behavior and Strength Characteristics in Sn-4.0wt%Ag-0.5wt%Cu Solder Joint Interface

)Abstract The micro-structural changes, strength characteristics, and micro-fractural behaviors at the jointinterface between a Sn-4.0wt%Ag-0.5wt%Cu solder ball and UBM treated by isothermal aging are reported.From the reflow process for the joint interface, a small amount of intermetallic compound was formed. Withan increase in the isothermal aging time, the type and amount of the intermetallic compound changed. Theinterface without an isothermal treatment showed a ductile fracture. However, with an increase in the agingtime, a brittle fracture occurred on the interface due mainly to the increase in the size of the intermetalliccompounds and voids. As a result, a drastic degradation in the shear strength was observed. From a micro-shear test by a scanning electron microscope, the generation of micro-cracks was initiated from the voids atthe joint interface. They propagated along the same interface, resulting in coalescence with neighboring cracksinto larger cracks. With an increase in the aging time, the generation of the micro-structural cracks wasenhanced and the degree of propagation also accelerated.Key wordsBGA solder ball, solder joint interface, intermetallic compound(IMC), micro-fracture behavior,in-situ observation

The influences of AlxGa1-xN layer on the characteristics of UV LED structure

Abstract We have studied growth characteristics of AlxGa1−xN/GaN epilayers which were grown with various x values and their effects on the characteristics of UV LED chips fabricated by AlxGa1−xN/GaN double heterojunction having different band offset. The van der Pauw technique, double crystal X-ray diffractometry (DCXRD), optical microscope and photoluminescence (PL) were used to characterize crystallographic, electrical and optical properties of each AlxGa1−xN epilayer. The device characteristcs of the fabricated UV LED chips were evaluated by measuring current – voltage (I–V) and light power–current (L–I) measurements were carried out. The PL spectra and the DCXRD results show that the dislocation density of AlxGa1−xN epilayer resulted from the difference of lattice mismatch between AlxGa1−xN and GaN and the FWHM increases by raising the Al incorporation. The turn-on voltage of the UV LED chips fabricated by AlxGa1−xN/GaN double heterojunction is less dependent on both the x value of AlxGa1−xN cladding layer and the related band offset (ΔEg). It was found that the optical power is strongly dependent on the dislocation density of each AlxGa1−xN cladding layer related to the individual x value.

Improvements in Optical Power and Emission Angle of Blue Light Emitting Diodes Using Patterned Sapphire Substrates with Low Threading Dislocation Densities

GaN blue light-emitting diodes (LEDs) with a peak emission wavelength of approximately 461 nm were fabricated on a c-face lens patterned sapphire substrate (PSS) and an unpatterened sapphire substrate (UPSS) by metal organic chemical vapor deposition (MOCVD). The crystal structure of an epitaxial GaN film was improved by using the PSS. The peak wavelengths with electroluminescence intensities of 462 and 464 nm were measured for PSS and UPSS LEDs using a 20 mA injection current. It was found that the electroluminescence intensity of the LEDs grown on the lens PSS was about 61% higher than that of the LEDs prepared on the UPSS. The emission angle of the PSS LED increased by 1.5 times compared with that of the UPSS LED. The luminous intensities of both LEDs increased similarly on both circular and square reflectors. The output power of the PSS LED was 33% greater than that of the LED UPSS. In addition, the reduction in full width at half maximum (FWHM) in the ω-scan rocking curves of GaN on the PSS suggested an improved crystal quality. These significant improvements in output power and emission angle resulted from the enhanced light extraction and the reduced threading dislocation (TD) density using the PSS method.

Experimental and computational analyses of electroabsorption in polar InGaN/GaN quantum zigzag heterostructures

Traditional quantum confined Stark effect is well known to lead to strong electroabsorption in multiple quantum well (MQW) structures, yielding only red-shift of the absorption edge with the externally applied electric field, independent of the direction of the applied field. However, a little is known the electroabsorption behavior in III nitride quantum structures grown on c-plane of their wurtzite crystal structure, which is substantially different than the electroabsorption of conventional quantum structures. Such III-N heterostructures exhibit strong polarization fields and discontinuity of such polarization fields at their heterointerfaces causes stimulation of large electrostatic fields in alternating directions for their wells and barriers. Consequently, their energy band diagrams form a zigzag potential profile in conduction and valence bands, instead of those with square profiles. A natural and suitable approach for understanding these polarization fields and also developing insight to design related devices (e.g., electroabsorption modulators) is to study electroabsorption behavior as a function of the polarization field in such polar structures. To this end, we present a comparative, computational and experimental study of electroabsorption in our different designs of c-plane grown polar InGaN/GaN quantum structures with varying levels of polarization.