Dong Chul Choo

Carrier density and mobility modifications of the two-dimensional electron gas due to an embedded AlN potential barrier layer in AlxGa1−xN∕GaN heterostructures

The variations in the electronic properties of the two-dimensional electron gas (2DEG) in AlxGa1−xN∕GaN heterostructures due to an AlN embedded spacer layer were investigated by using Shubnikov–de Haas (SdH) measurements. The carrier densities of the 2DEGs in the Al0.4Ga0.6N∕AlN∕GaN and the Al0.4Ga0.6N∕GaN heterostructures, determined from the SdH data, were 8.75×1012 and 8.66×1012cm−2, respectively. The electron carrier density and the mobility of the 2DEG in the AlxGa1−xN∕GaN heterostructure with an AlN spacer layer were larger than those in the AlxGa1−xN∕GaN heterostructure. The electronic subband energies, the wave functions, and the Fermi energies in the Al0.4Ga0.6N∕AlN∕GaN and Al0.4Ga0.6N∕GaN heterostructures were calculated by using a self-consistent method taking into account spontaneous and piezoelectric polarizations. These present results indicate that the electronic parameters of the 2DEG occupying an AlxGa1−xN∕GaN heterostructure are significantly affected by an AlN spacer layer, and they can...

Efficiency Stabilization in Blue Organic Light-Emitting Devices Fabricated Utilizing a Double Emitting Layer with Fluorescence and Phosphorescence Doped Layers

The luminance efficiency of the blue organic light-emitting devices (OLEDs) fabricated utilizing a double emitting layer (DEML) with a 4,4′-Bis(2,2-diphenyl-ethen-1-yl)diphenyl (DPVBi) layer doped with 4,4′-Bis[4-(diphenylamino)styryl]biphenyl (BDAVBi) fluorescence dopant and a 4,4′-Bis(carbazol-9-yl)biphenyl (CBP) layer doped with a bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium III (FIrpic) phosphorescence dopant at 20 mA/cm2 was 6.2 cd/A, indicative of highly efficient OLEDs. Electroluminescence spectra for the OLEDs with a DEML showed that a dominant peak at 469 nm corresponding to the BDAVBi doped DPVBi layer together with a shoulder at 491 nm related to the combination of the BDAVBi doped DPVBi layer and the FIrpic doped CBP layer appeared.

Strain effects, electronic parameters, and electronic structures in modulation-doped InxGa1−xAs/InyAl1−yAs coupled step-rectangular quantum wells

The electronic parameters of a two-dimensional electron gas (2DEG) in unique modulation-doped InxGa1−xAs/InyAl1−yAs coupled step-rectangular quantum wells were investigated by using the Shubnikov–de Haas (SdH)–Van der Pauw Hall effect and cyclotron resonance measurements. The SdH measurements and the fast Fourier transformation results for the SdH data at 1.5 K indicated electron occupation of two subbands in the quantum well. The electron effective masses of the 2DEG were determined from the cyclotron resonance measurements, and their values qualitatively demonstrated the nonparabolicity effects of the conduction band on the 2DEGs in the quantum wells. The electronic subband energies, the energy wave functions, and the Fermi energies were calculated by using a self-consistent method taking into account exchange-correlation effects, together with strain and nonparabolicity effects. These results can help in understanding potential applications of unique InxGa1−xAs/InyAl1−yAs coupled step-rectangular quant...

Enhancement of the Hole Injection and Hole Transport in Organic Light Emitting Devices Utilizing a 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyano-quinodimethane Doped Hole Transport Layer

While the current densities of hole only devices with a 2,3,5,6-tetrafluoro-7,7,8, 8-tetracyano-quinodimethane (F4-TCNQ) doped N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) hole transport layer (HTL) slightly changed with increasing F4-TCNQ doping concentration, those of hole only devices with a F4-TCNQ doped 4,4′,4″-tris(N-(2-naphthyl)-Nphenylamino)triphenylamine (2-TNATA) HTL significantly increased. The hole injection and hole transport of hole only devices were enhanced by inserting an ultra thin F4-TCNQ layer between an indium-tin-oxide layer and a NPB HTL or a 2-TNATA HTL, regardless of the HTL materials. These results indicate that the hole injection and hole transport in OLEDs utilizing a F4-TCNQ doped HTL or a F4-TCNQ thin layer is enhanced.

Enhancement Mechanisms of the Color Purity in Blue Organic Light-Emitting Devices Fabricated Utilizing an Emitting Layer Containing Mixed Fluorescence and Phosphorescence Host Materials

The electrical and optical properties of blue organic light-emitting devices (OLEDs) containing a mixed host emitting layer (EML) consisting of a 1,3-bis(carbazole-9-yl)benzene (mCP) layer and a 3-tert-butyl-9,10-di(naphtha-2-yl)anthracene (TBADN) layer were investigated. The driving voltage of the OLEDs with a mixed host EML was smaller than that of the OLEDs with a single EML. The electroluminescence spectra for OLEDs containing a mixed host EML showed a dominant peak related to the mCP or the TBADN layer. The color coordinates of the OLEDs containing a 5% TBADN-doped mCP EML were (0.146, 0.091), indicative of the deep blue color coordinates.

Luminence Efficiency and Color Stabilization of Blue Organic Light-Emitting Devices Fabricated Utilizing a 4,7-diphenyl-1,10-phenanthroline Electron Transport Layer Containing an Ultra Thin Trap Layer

The hole blocking barrier in organic light-emitting devices (OLEDs) fabricated utilizing a 4,7-diphenyl-1,10-phenanthroline (BPhen) electron transport layer (ETL) with an ultra thin trapping layer was affected by the 5,6,11,12-tetraphenylnaphthacene (rubrene) or the tris(8-hydroxyquinoline) (Alq3) trapping layer, acting as a trapping layer in the ETL. While the rubrene layer in the ETL decreased the number of electrons and holes in the emitting layer (EML), resulting in a decrease in the current density and the luminence of the OLEDs, the Alq3 layer in the BPhen ETL did not block effectively the holes transfer from the EML to the ETL, resulting in a decrease in the luminence efficiency. The electroluminescence spectra showed that the magnitude of the holes blocked by the ETL was not significantly affected by the thin rubrene layer embedded in the BPhen ETL.

Equivalent Circuit Models in Organic Light-Emitting Diodes Designed Using a Cole-Cole Plot

The equivalent circuit models in organic light-emitting diodes (OLEDs) consisting of indium-tin-oxide (ITO)/tris(8-hydroxyquinoline) Aluminum (Alq3)/Aluminum (Al), ITO/Alq3/Lithium quinolate/Al or ITO/N,N-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/Alq3/Al were investigated. The relaxation times were obtained from the impedances as functions of frequencies, and the equivalent circuits in the OLEDs were designed by using the relaxation times of OLEDs determined from the impedance spectroscopies. The Cole-Cole plots calculated from the equivalent circuit models were in reasonable agreements with the experimental results. These results can help designing equivalent circuit models of multilayer OLEDs.

Luminance Efficiency Enhancement in Organic Light-Emitting Devices Utilizing 4,7-Diphenyl-1,10-phenanthroline/aluminum tris(8-hydroxyquinoline) Multiple Heterostructures Acting as an Electron Transport Layer

The electrical and optical properties of organic light-emitting devices (OLEDs) with three periods of 4,7-diphenyl-1,10-phenanthroline (BPhen)/aluminum tris(8-hydroxyquinolate) (Alq3) multiple heterostructures acting as an electron transport layer (ETL) were investigated. While the leakage current of OLEDs with multiple heterostructures was smaller than that of OLEDs without multiple heterostructures, the luminance efficiency was larger than that of OLEDs without multiple heterostructures. The BPhen layers in the multiple heterostructures blocked holes from the emitting layer (EML) to the ETL, and they enhanced the electron injection from the cathode to the EML, resulting in an increase in the luminance efficiency.

Enhancement of Luminance Efficiency in Green Organic Light-Emitting Devices Utilizing a Cesium Nitrate/Lithium Quinolate Electron Injection Layer

The current density and the luminance efficiency of the organic light-emitting devices (OLEDs) with a cesium nitrate (CsNO3)/lithium quinolate (Liq) electron injection layer (EIL) were larger than those of the OLEDs with a Liq EIL. Residual Cs ions or cesium oxides in the OLEDs decreased the electron affinity of the cathode electrode, resulting in an improvement of the electron injection efficiency and the luminance efficiency. Electron only devices, which are elements being able to flow only electrons, showed that the electron injection magnitude was increased due to the insertion of the CsNO3 EIL into OLEDs.

Enhancement of Luminance Efficiency in Organic Light-Emitting Devices Utilizing Mixed Layers Embedded in Electron Transport Layers

While the hole transport decreased in the mixed layer in the organic light-emitting devices (OLEDs) with an Al:lithium quinolate (Liq) mixed layers in the electron transport layer (ETL), the electron injection increased the Al:Liq mixed layer. The enhancement of luminance efficiency in the OLEDs with an Al:Liq mixed layer in the ETL originated from the more balance between the electrons and the holes in the emitting layer due to the decrease in the hole injection and the increase in the electron injection resulting from the existence of the Al:Liq mixed layer.