Hee-Baik Chae

Electron mobility in tris(8-hydroxyquinoline)aluminum (Alq3) films by transient electroluminescence from single layer organic light emitting diodes

Single layer devices of indium tin oxide/Alq3∕Al were constructed with varying the active areas from 1to8mm2 and the thicknesses from 30to50nm. Average electric field across the Alq3 layer during the transient state was estimated from the accumulated charges at the interfaces of the devices. The electron mobility could thus be calculated by assuming that the injected charge carriers moved under the average electric field rather than the instantaneous field. The resulting mobility could be determined uniquely in a device thickness. The electron drift mobility was shown to behave similarly to the time-of-flight results.

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 υm films is 9 and 16 W-cm−1. K−1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.

Thermal Diffusivity of Diamond Wafers Deposited with Multicathode dc Plasma-Assisted CVD

A thermal diffusivity map for diamond wafers of 10-cm diameter was obtained using a converging thermal wave technique in a nondestructive and noncontact manner. Diamond wafers were deposited by seven-cathode dc plasma-assisted chemical vapor deposition with different CH4concentrations in pure hydrogen and applied powers of the plasma. Six cathodes were located at the apexes of a hexagon with an arm distance of 4.3 cm about a central cathode. The wafer deposited at a low-power plasma (13.47 kW) and a low concentration of CH4(6%, by volume) shows three circular zones on the thermal diffusivity map. The thermal diffusivity shows the lowest value at the center. It increases to about 10% in a radius of 2 to 3 cm and then decreases with further increases in the radius. The optical photograph and the Raman lines of the wafer show patterns similar to those of the thermal diffusivity. These are affected by the locations of the cathodes in the deposition chamber when the plasma power is low. Diamond wafers deposited at a high-power plasma (20.58 kW) with high concentrations of methane (10%, by volume) show higher values of thermal diffusivity and better uniformity than wafers deposited at a low power and low methane concentration. A fine crack can be located on a wafer with the converging thermal wave technique.

Measurements of Thermal Diffusivity for Thin Slabs by a Converging Thermal Wave Technique

The measurement of thermal diffusivity for thin slabs by a converging thermal wave technique has been studied. Temperature variation at the center of the heat source ring that is produced by a pulsed high-power laser is detected by an infrared detector. A computer program based on the finite difference method is developed to analyze the thermal diffusivity of the slabs. Materials of both high thermal diffusivity (CVD diamond wafer) and low thermal diffusivity (stainless-steel foil) have been used for the measurements. The measurements have been performed by varying the size and the thickness of specimen. The converging thermal wave technique has proved to be a good method to measure the thermal diffusivity of a CVD diamond without breaking the wafer into small specimens. The technique can be applied for a small slab if the diameter of the slab is two times larger than that of the heat source ring. The sensitivity of thickness in measuring the thermal diffusivity is low for ordinary CVD diamond. The use of the converging thermal wave technique for nonhomogeneous, nonuniform, and anisotropic materials has been accomplished by applying the finite difference method.

Thermal diffusivity measurements of foil-shaped materials by vectorial analysis using an AC calorimeter

Measurements of thermal diffusivity of foil-shaped materials have been carried out using a photoirradiation-type AC calorimeter at room temperature. In this method the frequency effect, which is caused by heat loss from the sample to the environment, is readily detected in measurements of both amplitude and phase components of the AC temperature signal, Even though the chopping frequency is appropriate, the two diffusivities calculated from these two components differ from each other, Moreover, the difference between the two values increases when the chopping frequency increases. Simple vectorial calculation with the two components one from the amplitude and the other from the phase- permits the frequency ellect to be determined. The calculated result is the geometric average of the two diffusivities. This analytic method was tested with diamond film and SUS-304 foil. From these we confirmed that the vectorial analytic method gives similar diffusivity values for different frequencies indicating its reliability.

Thermal conductivity of dc‐plasma assisted chemical vapor deposited diamond films

The dc‐plasma assisted chemical vapor deposition method has been used to synthesize diamond films. Thermal diffusivity of these films has been measured in 120–800 K with a modified Angstrom method. Phonon scattering processes are considered to analyze thermal conductivity with the full Callaway model. In analysis, microstructure of grain boundaries and extended defect concentration give significant effects to the mean free path of phonons in low temperatures. At high temperatures, the thermal conductivity is governed by the intrinsic thermal resistive process, the umkalpp process. Thermal conductivity of the films above 500 K is shown to close to a recent measurement of natural diamond. This supports that the crystal structure of the films is not different with the bulk diamond.