Ki-Bum Kim

Homogeneous dispersion of organic p-dopants in an organic semiconductor as an origin of high charge generation efficiency

We report that an organic p-dopant tri[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] [Mo(tfd)3] resulted in higher density of holes than inorganic metal oxide dopants of ReO3 or MoO3 in 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine even though the metal oxide dopants possess deeper work functions compared to Mo(tfd)3. Higher charge generation efficiency results largely from the homogeneous dispersion of Mo(tfd)3 in the host. In contradistinction, the transmission electron microscopy analysis revealed a formation of metal oxide nanoclusters. This highlights the importance of homogeneous dispersion for an efficient doping.

Effect of Al Distribution on Carrier Generation of Atomic Layer Deposited Al-Doped ZnO Films

The effect of the Al distribution on the electrical properties of Al-doped ZnO (AZO) films deposited by atomic layer deposition (ALD) is investigated. In order to control the Al distribution, the pulsing time of trimethylaluminum (TMA) is varied from 2 (within an ALD window) to 0.1 s. As a result, the areal density of Al atoms incorporated in a single dopant layer decreases from 3.3 x 10 14 to 1.2 ×10 14 cm -2 . Hall measurements reveal that the minimum resistivity of the ALD-AZO films is decreased from 3.2 x 10 -3 to 1.7 ×10 -3 Ω cm as a result of reducing the TMA pulsing time from 2 to 0.1 s. This decrease is due to the obvious increase of the carrier concentration from 1.4 x 10 20 to 4.7 x 10 20 cm -3 . It is suggested that both the improved doping efficiency (from 13 to 58%) and the insertion of more dopant layers within the ZnO matrix are responsible for the increase of the carrier concentration.

Theoretical and experimental study of nanopore drilling by a focused electron beam in transmission electron microscopy

Sub-10 nm nanopores drilled by a focused electron beam in a transmission electron microscope are widely used in solid-state nanopore devices for DNA translocation. However, there still remains much controversy surrounding the drilling mechanism. In order to explain the drilling of nanopores by electrons, we undertook a theoretical consideration of the energy transfer from the fast electrons to the solid through such mechanisms as elastic and inelastic scattering. According to the calculations based on the scattering cross-section, the direct atomic displacement cross-section induced by elastic scattering increases with increasing incident electron energy, while the ionization cross-section and temperature increment decrease. We performed nanopore drilling in a Si3N4 membrane using two different electron energies, 200 and 300 kV, to identify the drilling mechanism. The dependence of the nanopore drilling on the incident electron energy was well matched with the direct atomic displacement.

Effect of surface tension and coefficient of thermal expansion in 30 nm scale nanoimprinting with two flexible polymer molds

We report on nanoimprinting of polymer thin films at 30xa0nm scale resolution using two types of ultraviolet (UV)-curable, flexible polymer molds: perfluoropolyether (PFPE) and polyurethane acrylate (PUA). It was found that the quality of nanopatterning at the 30xa0nm scale is largely determined by the combined effects of surface tension and the coefficient of thermal expansion of the polymer mold. In particular, the polar component of surface tension may play a critical role in clean release of the mold, as evidenced by much reduced delamination or broken structures for the less polarized PFPE mold when patterning a relatively hydrophilic PMMA film. In contrast, such problems were not notably observed with a relatively hydrophobic PS film for both polymer molds. In addition, the demolding characteristic was also influenced by the coefficient of thermal expansion so that no delamination or uniformity problems were observed when patterning a UV-curable polymer film at room temperature. These results suggest that a proper polymeric mold material needs to be chosen for patterning polymer films under different surface properties and processing conditions, providing insights into how a clean demolding characteristic can be obtained at 30xa0nm scale nanopatterning.

Vertically and Laterally Self-Aligned Double-Layer of Nanocrystals in Nanopatterned Dielectric Layer for Nanocrystal Floating Gate Memory Device

The formation of a vertically and laterally self-aligned double layer of CdSe colloidal nanocrystals (NCs) in a nanopatterned dielectric layer on Si substrate was demonstrated by a repeating dip-coating process for NC deposition and atomic layer deposition (ALD) of Al 2 O 3 layer. A nanopatterned SiO 2 /Si substrate was formed by patterning with a self-assembled diblock copolymer. After the selective deposition of the first NC layer inside the SiO 2 nanopattern by dip-coating, an Al 2 O 3 interdielectric layer and the second NC layer in the Al 2 O 3 nanopattern were sequentially deposited. The capacitance-voltage measurement of an Al-gate/ALD-Al 2 O 3 (25 nm)/second-Cdse-NCS/ALD-Al 2 O 3 (2 nm)/first-CdSe-NCs/nanopatterned-SiO 2 (15 nm)/p-Si substrate structure showed the flatband voltage shift through the charge transport between the gate and NCs.

Resistive switching characteristics of maghemite nanoparticle assembly

The resistive switching characteristics of the assembly of maghemite (γ-Fe2O3) nanoparticles having a diameter of ∼10nm were investigated in the structure of Al/γ-Fe2O3 nanoparticle multilayer (∼300nm thick)/Al-plate. The nanoparticle multilayer on Al plate was formed by repeating dip-coating processes. The multilevel (five states) resistive switching was observed with the resistance values ranging from ∼4.8 ×10 5 to 2.7 ×10 3 � depending on the externally applied voltage. The multilevel switching is thought to originate from the repetitive and reversible formation and rupture of multiple conducting filaments. It demonstrates the potential application of the γ-Fe2O3 nanoparticle assembly for resistive switching devices. (Some figures in this article are in colour only in the electronic version)

New Method of Evaluating the Crystallization Activation Energy of Ge2Sb2Te5 by In situ Resistance Measurement

A new method of evaluating crystallization activation energy of Ge2Sb2Te5 is proposed by in situ resistance measurement under isothermal annealing conditions. Linear relationship between logarithmic time and reciprocal temperature in modified Johnson–Mehl–Avrami–Kolmogorov equation is derived under the assumption that proportion of resistance drop from the initial value is closely related to crystal fraction. Crystallization activation energy thus obtained is 2.67 eV. Numerical calculation was conducted to manifest the validity of this analysis based on percolation model. Moreover, crystallization behavior of patterned single-line structure of Ge2Sb2Te5 was evaluated, and the scaling effect of increasing activation energy with decreasing line width was observed.

Controlling spatial density and size of nanocrystals by two-step atomic layer deposition

Two-step atomic layer deposition (ALD) is proposed in order to control both the spatial density and size of nanocrystals (NCs) via modulation of the nucleation rate during deposition. In this process, two different deposition conditions are sequentially used: a high nucleation rate condition for the formation of high density NCs and a low nucleation rate condition with a slow growth rate for the subsequent growth of pre-formed NCs. To control the nucleation rate of Ru during ALD, pulsing time and carrier flow rate of the Ru precursor are varied. By controlling those factors, both the film growth rate and a nucleation rate of Ru are decreased considerably. Two-step ALD of Ru NCs using the surface-saturated condition followed by the reduced condition allows for variation of the spatial density from 7.9 × 10(11) to 3.2 × 10(12) cm(-2) and variation of the average diameter from 1.9 to 3.3 nm.