Dae Hong Ko

Determination of elastic modulus and Poisson's ratio of diamond-like carbon films

A simple technique to measure the elastic modulus and Poisson’s ratio of diamond-1ike carbon (DLC) films deposited on Si substrate was suggested. This technique involved etching a side of Si substrate using the DLC film as an etching mask. The edge of the DLC overhang, which is free from constraint of the Si substrate, exhibits periodic sinusoidal shape. By measuring the amplitude and the wavelength of the sinusoidal edge, we can determine the strain of the film required to adhere to the substrate. Combined with an independent stress measurement by laser reflection method this technique allows calculation of the biaxial elastic modulus, E=O1 2 nU where E is the elastic modulus and n Poisson’s ratio of the DLC films. By comparing the biaxial elastic modulus with plane-strain modulus E=O1 2 n 2 U measured by nanoindentation, we could further determine the elastic modulus and Poisson’s ratio, independently. The mechanical properties of DLC films deposited by r.f. PACVD were characterized using this technique. The films were prepared by using C6H6 r.f. glow discharge at a self bias voltage of 400 V and a deposition pressure of 1.33 Pa. The elastic modulus and Poisson’s ratio were 87 ^ 18 GPa and 0:22 ^ 0:33, respectively. The effects of the etching depth and the film thickness were also discussed. q 1999 Elsevier Science S.A. All rights reserved.

Effect of Doped Nitrogen on the Crystallization Behaviors of Ge2Sb2Te5

The crystallization behaviors of Ge 2 Sb 2 Te 5 and nitrogen-doped Ge 2 Sb 2 Te 5 films, deposited by dc magnetron sputtering, were investigated using the in situ resistance measurement, X-ray diffractometry, and transmission electron microscopy (TEM). The kinetic constants were estimated using the classical Johnson-Mehl-Avrami-Kolmogrov model from the resistance measurement of Ge 2 Sb 2 Te 5 and nitrogen-doped Ge 2 Sb 2 Te 5 films. The nitrogen-doped Ge 2 Sb 2 Te 5 showed a one-step process with the Avrami constant (n) of 1, while the Ge 2 Sb 2 Te 5 thin film showed a two-step transformation with n of 3.6 and 1. The cross-sectional TEM revealed that the crystallizations were concentrated to the regions of 20-30 nm near the top surfaces rather than occurring throughout the films of Ge 2 Sb 2 Te 5 and nitrogen-doped Ge 2 Sb 2 Te 5 films. The annealed Ge 2 Sb 2 Te 5 showed continuous crystalline films with 20 nm grains at the surface in conjunction with the saturation of the sheet resistance, while the annealed nitrogen-doped Ge 2 Sb 2 Te 5 showed nanocrystalline phases with 5 nm grains near the surface with a continuous decrease in sheet resistance. The crystallization of nitrogen-doped Ge 2 Sb 2 Te 5 was observed to be a nucleation-dominant process, while that of Ge 2 Sb 2 Te 5 was a growth-dominant process.

Effect of In incorporated into SbTe on phase change characteristics resulting from changes in electronic structure

The effects of the In content of InSbTe films with various stoichiometries (Sb2Te2.7, In0.5Sb2Te2.9, and In2.6Sb2Te2.9) on phase change characteristics were investigated. With increasing incorporation of In atoms into Sb2Te3, various crystalline phases, i.e., In2Te3, Sb, and In3SbTe2, were observed due to the bond energy between the constituent atoms, while only Sb2Te3 and the Sb2Te2 phases were observed in the case of Sb2Te2.7 and In0.5Sb2Te2.9 films. In addition, the shifts in binding energy of the Sbu20023d and Inu20023d peaks in x-ray photoelectron spectra after the annealing treatment were directly related to the amount of incorporated In. The observed changes in electronic structure suggest that the changes in electrical conductivity and crystalline phase are directly related to the extent of In incorporation.

Effects of Nitrogen Incorporation in HfO2 Grown on InP by Atomic Layer Deposition: An Evolution in Structural, Chemical, and Electrical Characteristics

We investigated the effects of postnitridation on the structural characteristics and interfacial reactions of HfO2 thin films grown on InP by atomic layer deposition (ALD) as a function of film thickness. By postdeposition annealing under NH3 vapor (PDN) at 600 °C, an InN layer formed at the HfO2/InP interface, and ionized NHx was incorporated in the HfO2 film. We demonstrate that structural changes resulting from nitridation of HfO2/InP depend on the film thickness (i.e., a single-crystal interfacial layer of h-InN formed at thin (2 nm) HfO2/InP interfaces, whereas an amorphous InN layer formed at thick (>6 nm) HfO2/InP interfaces). Consequently, the tetragonal structure of HfO2 transformed into a mixture structure of tetragonal and monoclinic because the interfacial InN layer relieved interfacial strain between HfO2 and InP. During postdeposition annealing (PDA) in HfO2/InP at 600 °C, large numbers of oxidation states were generated as a result of interfacial reactions between interdiffused oxygen impurities and out-diffused InP substrate elements. However, in the case of the PDN of HfO2/InP structures at 600 °C, nitrogen incorporation in the HfO2 film effectively blocked the out-diffusion of atomic In and P, thus suppressing the formation of oxidation states. Accordingly, the number of interfacial defect states (Dit) within the band gap of InP was significantly reduced, which was also supported by DFT calculations. Interfacial InN in HfO2/InP increased the electron-barrier height to ∼0.6 eV, which led to low-leakage-current density in the gate voltage region over 2 V.

Effects of Interface Al2O3 Passivation Layer for High-k HfO2 on GaAs

Effects of Interface Al2O3 Passivation Layer for High-k HfO2 on GaAs Dong Chan Suh,* Young Dae Cho,* Dae-Hong Ko, Yongshik Lee, Kwun Bum Chung, and Mann-Ho Cho Department of Materials Science and Engineering, Department of Electrical and Electronics Engineering, and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea Department of Physics, Dankook University, Cheonan 330-714, Korea

Crystallization Behaviors of Laser Induced Ge2Sb2Te5 in Different Amorphous States

We examined the crystallization behavior of Ge 2 Sb 2 Te 5 in different amorphous states in the context of the kinetic approach using a laser heating method. We measured the changes in reflectance values after varying the applied laser pulse power and duration time and estimated the fraction of the crystalline phase of Ge 2 Sb 2 Te 5 with a nanosecond time scale. The modification of the laser pulse sequence allowed a comparison between the crystallization behaviors of the as-deposited and melt-quenched amorphous states. We determined crystallization kinetics, such as nucleation and growth mechanisms, by combining the results from reflectance measurements and transmission electron microscopy. We observed the different crystallization behaviors between the as-deposited and melt-quenched Ge 2 Sb 2 Te 5 films. The melt-quenched Ge 2 Sb 2 Te 5 demonstrated fast crystallization with a short incubation time compared to the as-deposited sample. The melt-quenched Ge 2 Sb 2 Te 5 also crystallized with a number of fine grains in contrast with the few larger grains seen in the as-deposited Ge 2 Sb 2 Te 5 .

Filament Geometry Induced Bipolar, Complementary, and Unipolar Resistive Switching under the Same Set Current Compliance in Pt/SiOx/TiN.

The decidedly unusual co-occurrence of bipolar, complementary, and unipolar resistive switching (BRS, CRS, and URS, respectively) behavior under the same high set current compliance (set-CC) is discussed on the basis of filament geometry in a Pt/SiOx/TiN stack. Set-CC-dependent scaling behavior with relations Iresetu2009~u2009R0–α and Vresetu2009~u2009R0–β differentiates BRS under low set-CC from other switching behaviors under high set-CC due to a low α and β involving a narrow filamentary path. Because such co-occurrence is observed only in the case of a high α and β involving a wide filamentary path, such a path can be classified into three different geometries according to switching behavior in detail. From the cyclic switching and a model simulation, we conclude that the reset of BRS originates from a narrower filamentary path near the top electrode than that of CRS due to the randomness of field-driven migration even under the same set-CC. Also, we conclude that URS originates from much narrower inversed conical filamentary path. Therefore, filament-geometry-dependent electric field and/or thermal effects can precisely describe the entire switching behaviors in this experiment.

Strain characterization of fin-shaped field effect transistors with SiGe stressors using nanobeam electron diffraction

This study undertook strain analysis on fin-shaped field effect transistor structures with epitaxial Si1−xGex stressors, using nano-beam electron diffraction and finite elements method. Combining the two methods disclosed dynamic strain distribution in the source/drain and channel region of the fin structure, and the effects of dimensional factors such as the stressor thickness and fin width, offering valuable information for device design.

In0.7Ga0.3As quantum well MOSFETs with Al2O3/HfO2 toward subthreshold swing of ∼60 mV/dec

We have successfully demonstrated In0.7Ga0.3As quantum well (QW) MOSFETs with Al2O3/HfO2 toward a subthreshold swing of ~60 mV/decade. The fabricated In0.7Ga0.3As QW MOSFET with Lg = 5 µm exhibits an excellent subthreshold swing of 69 mV/dec and a drain-induced barrier lowering (DIBL) of less than 10 mV/V at VDS = 0.5 V with EOT ~0.8 nm. On the basis of measured C–V and I–V data, we extracted the effective mobility (μn,eff) from our long-channel InGaAs QW MOSFET, yielding an excellent μn,eff of approximately 3,400 cm2 V−1 s−1 at 300 K.

Relaxation of misfit strain in silicon-germanium (Si1−xGex) films during dry oxidation

The effects of oxidation on strain relaxation in Si1−xGex layers on silicon substrates were investigated. Si1−xGex layers, with different Ge fractions (x=0.15 and 0.3), were grown on chemically cleaned silicon substrates by an ultrahigh vacuum chemical vapor deposition process. Oxidation at 800 and 900u2009°C under O2 ambient in a tube furnace resulted in the production of silicon oxide layers on top and a Ge-rich region in the Si1−xGex films. It was observed that the oxidation of Si0.85Ge0.15 films at 900u2009°C produced the relaxation of the misfit strain in the remnant Si1−xGex layer and the increase in strain in the Ge pile-up layer with increasing oxidation time, while the oxidation at 800u2009°C produced no changes in the misfit strain in the Si1−xGex layers. The oxidation of Si0.70Ge0.30 films at 800u2009°C showed the relaxation of the misfit strain in the remnant Si1−xGex layer with the accumulation of stain in the Ge pile-up layer. However, the oxidation of Si0.70Ge0.30 layers at 900u2009°C exhibited the strain rela...