Tak Jeong

Transfer of GaN LEDs From Sapphire to Flexible Substrates by Laser Lift-Off and Contact Printing

We fabricate flexible GaN-based light-emitting diode (LED) systems by laser lift-off (LLO) and transfer printing methods. LLO enables transferring a whole GaN LED layer from sapphire onto a silicon handling wafer to provide a stable platform for any shape of LED. Polymer pedestal structures underneath the LEDs support efficient transfer printing of the patterned LED array from the silicon handling wafer to a flexible substrate. We demonstrate the efficacy of the technique by presenting 9 × 9 LED arrays on polyethylene terephthalate and heart-shaped LED pixels on a piece of paper with transfer yields of 92% and 79%, as well as their successful illumination.

Origin of point defects in AgInS2/GaAs epilayer obtained from photoluminescence measurement

The AgInS2 epilayers with a chalcopyrite structure grown using a hot-wall epitaxy method have been confirmed to be a high quality crystal. From the optical absorption measurement, the temperature dependence of the energy band gap on AgInS2/GaAs was found to be Eg(T)=2.1365 eV−(9.89×10−3 eV)T2/(2930+T). After the as-grown AgInS2/GaAs was annealed in AgInS2/GaAs has been investigated by using the photoluminescence (PL) at 10 K. The native defects of VAg, VS, Agint, and Sint obtained from PL measurements were classified as a donors or acceptors type AgInS2/GaAs to an optical p type. Also, we confirmed that In in AgInS2/GaAs did not form the native defects because In in AgInS2 did exist in the form of stable bonds.

High-Brightness GaN-Based Light-Emitting Diodes on Si Using Wafer Bonding Technology

GaN-based light-emitting diodes (LEDs) grown on Si(111) substrates were fabricated with a vertical electrode method by using wafer bonding technology. The fabricated vertical LEDs showed a lower operating voltage and larger light output power than conventional LEDs due to enhancement in current spreading and reduction in tensile strain. The light output power of the vertical structured LEDs was 2.6 times higher than that of conventional LEDs, with an operating voltage at 20 mA reduced from 3.5 to 3.2 V.

InGaN/AlGaN Ultraviolet Light-Emitting Diode with a Ti3O5/Al2O3 Distributed Bragg Reflector

Here, we report InGaN/AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using distributed Bragg reflectors (DBRs). The DBRs consist of 11 layers of alternating quarter-wave thickness Ti3O5 and Al2O3 deposited onto the indium–tin-oxide ohmic contact layer of a 385 nm UV LED by ion-assisted electron beam evaporation. Numerical calculations showed that the angle averaged reflectivity of the GaN/DBR structure at 385 nm was 12% higher than that of a conventional GaN/Ag structure. The measured light output–current–voltage of the DBR UV LEDs at 20 mA current injection showed an increase of 15% in output power in comparison to a conventional Ag reflector with no degradation in electrical properties. We attributed this light output enhancement to the increased light extraction from the higher DBR reflectivity.

Enhanced Light Output Power of GaN-Based Vertical Light-Emitting Diodes by Using Highly Reflective ITO–Ag–Pt Reflectors

Highly reflective and thermally stable indium-tin-oxide (ITO)-Ag-Pt p-type reflectors for use in high-performance GaN-based light-emitting diodes (LEDs) have been investigated. The specific contact resistance of the ITO-Ag-Pt contacts was found to be 7.2 ×10-5Omegamiddotcm2. The ITO-Ag-Pt contacts showed a higher reflectance after thermal annealing (82% at 460 nm), while the reflectance of the ITO-Ag contacts was reduced from 81% to 65%. In addition, surface agglomeration was drastically decreased, indicating that the Pt layer efficiently prevents the surface agglomeration of the Ag layer. The vertical LEDs (VLEDs) fabricated with the ITO-Ag-Pt contacts had a 17% higher output power (at 20 mA) than the VLEDs fabricated with the ITO-Ag contacts.

Point defects in p-type CdIn2Te4 Bridgman grown crystals

Abstract Single crystal of p-CdIn 2 Te 4 was grown in a three-stage vertical electric furnace by using Bridgman method. The quality of the grown crystal has been investigated by X-ray diffraction and photoluminescence (PL) measurements. From the PL spectra of the as-grown CdIn 2 Te 4 crystal and the various heat-treated crystals, the (D 0 , X) emission was found to be the dominant intensity in the PL spectrum of the CdIn 2 Te 4 :Cd, while the (A 0 , X) emission completely disappeared in the CdIn 2 Te 4 :Cd. However, the (A 0 , X) emission in the PL spectrum of the CdIn 2 Te 4 :Te was the dominant intensity like in the as-grown CdIn 2 Te 4 crystal. These results indicated that the (D 0 , X) is associated with V Te which acted as donor and that the (A 0 , X) emission is related to V Cd which acted as acceptor, respectively. The p-CdIn 2 Te 4 crystal was obviously found to be converted into n-type after annealing in Cd atmosphere. The origin of (D 0 , A 0 ) emission and its TO phonon replicas is related to the interaction between donors such as V Te or Cd int , and acceptors such as V Cd or Te int . Also, the In in the CdIn 2 Te 4 was confirmed not to form the native defects because it existed in a stable bonding form.

Three-Dimensional Porous Copper-Graphene Heterostructures with Durability and High Heat Dissipation Performance

Porous materials have historically been of interest for a wide range of applications in thermal management, for example, in heat exchangers and thermal barriers. Rapid progress in electronic and optoelectronic technology necessitates more efficient spreading and dissipation of the heat generated in these devices, calling for the development of new thermal management materials. Here, we report an effective technique for the synthesis of porous Cu-graphene heterostructures with pores of about 30 μm and a porosity of 35%. Graphene layers were grown on the surfaces of porous Cu, which was formed via the coalescence of molten Cu microparticles. The surface passivation with graphene layers resulted in a thermal conductivity higher than that of porous Cu, especially at high temperatures (approximately 40% at 1173 K). The improved heat dissipation properties of the porous structures were demonstrated by analysis of the thermal resistance and temperature distribution of LED chips mounted on the structures. The effective combination of the structural and material properties of porous Cu-graphene heterostructures provides a new material for effective thermal management of high-power electronic and optoelectronic devices.

Aluminum Nitride Ceramic Substrates-Bonded Vertical Light-Emitting Diodes

We confirmed the potential of an aluminum nitride (AlN) substrate to be used as a bonding material for the high current operation of vertical light-emitting diodes (VLEDs). For the electrical connection to the top and bottom of the AlN substrate, via-holes were formed by laser drilling and then filled with Ag, which plays a role in improving the thermal dissipation from the VLEDs. The forward voltage of the fabricated AlN-bonded VLEDs was 3.54 V at 350 mA, which is similar to that of the Si-bonded VLEDs. It was also found that the light output power of the AlN-bonded VLEDs increased steadily with increasing injection current up to 1 A, while that of the Si-bonded VLEDs started to decrease at around 850 mA. In addition, the thermal resistance of the AlN-bonded VLEDs was significantly reduced, as compared with that of the Si-bonded VLEDs and conventional LEDs, under the same package conditions.

Band gap energy and valence band splitting of p-CdIn2Te4 crystal by photocurrent spectroscopy

Single crystals of p-CdIn2Te4 were grown by the Bridgman method without a seed crystal. From photocurrent measurements, it was found that three peaks, A, B, and C, correspond to the intrinsic transition from the valence band states of Γ7(A), Γ6(B), and Γ7(C) to the conduction band state of Γ6, respectively. The crystal field splitting and the spin orbit splitting were found to be 0.2360 and 0.1119 eV, respectively. The temperature dependence of the CdIn2Te4 band gap energy was given by Eg(T)=Eg(0)−(9.43×10−3)T2/(2676+T). The Eg(0) was calculated to be 1.4750, 1.7110, and 1.8229 eV at the valence band states of Γ7(A), Γ6(B), and Γ7(C), respectively. The band gap energy of p-CdIn2Te4 at room temperature was determined to be 1.2023 eV.

High Efficiency InGaN Blue Light-Emitting Diode With

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.