Soo-Hyung Seo

Characterization of undoped and Cu-doped ZnO films for surface acoustic wave applications

Cu-doped ZnO (denoted by ZnO:Cu) films have been prepared by RF magnetron co-sputtering of a ZnO target with some Cu-chips attached. X-Ray diffraction (XRD) spectra of deposited ZnO:Cu films were measured and texture coefficient (TC) values for (002)-orientation were estimated. Optimal ranges of RF powers and substrate temperatures for obtaining high TC values were determined. Effects of Cu-doping conditions (such as Cu-chip sputtering areas and O2/(Ar+O2) mixing ratios) on TC values, electrical resistivities, and relative Cu-compositions of deposited films have been systematically investigated. X-Ray photoelectron spectroscopy (XPS) study suggests that the relative densities of metallic copper (Cu0) atoms and CuO (Cu2+)-phases within deposited films may play an important role in determining their electrical resistivities. Highly resistive (>1010 Ωcm) ZnO films with high TC values (>80%) can be achieved by Cu-doping. Surface acoustic wave (SAW) devices with ZnO:Cu (or ZnO)/interdigital transducer (IDT)/SiO2/Si configuration were also fabricated to estimate the effective electro-mechanical coupling coefficient (keff2) and insertion loss. The devices using Cu-doped ZnO films have higher keff2 and lower insertion loss, compared with those using undoped films.

A novel method of fabricating ZnO/diamond/Si multilayers for surface acoustic wave (SAW) device applications

Abstract A novel process for fabricating ZnO/diamond/Si for a surface acoustic wave device is as follows: to form a trench of 10 μm in depth, the Si wafer is chemically etched by employing the SiO2 layer as a mask. Selective growth of polycrystalline diamond film is carried out by a microwave plasma CVD using nominal conditions of 700 W microwave power, 40 torr pressure, 0.5% CH4/H2 ratio, 630 °C temperature, and −200 V bias enhancement for initial nucleation. After removing the SiO2 layer, indirect bonding of a Si handle wafer is performed at low temperatures of approximately 90 °C. Finally, the bottom Si wafer is mechano-chemically polished until the surface (backside) of the diamond is exposed. Raman and field emission SEM observations show that a high quality diamond film is selectively grown only on the trenched Si region. It has also been found from the AFM results that the backside surface roughness of the exposed diamond film is measured to be lower than 10 nm.

XPS and XRR studies on microstructures and interfaces of DLC films deposited by FCVA method

Diamond-like carbon (DLC) films have been deposited at room temperature using a filtered cathodic vacuum arc technique. The influence of the negative bias voltage applied to the substrate from 0 to −250 V on the sp3 hybridized carbon fraction is studied by Raman spectroscopy, which is also compared with the result obtained from X-ray photoelectron spectroscopy (XPS) for C 1s core peak. Based on the depth profiles for C 1s, Si 2p, and O 1s XPS peaks, the DLC film is modeled as a structure having three different layers, such as surface, bulk, and interface. In addition, the X-ray reflectivity is proposed as a method for estimating the density, surface roughness, and thickness of the layers constituting the DLC film. The estimated thickness of DLC film shows good agreement with the result obtained from the transmission electron microscope measurement.

Roughness control of polycrystalline diamond films grown by bias-enhanced microwave plasma-assisted CVD

Abstract A simple growth technique to control the surface roughness of polycrystalline diamond films is proposed. The films are grown using a microwave plasma-assisted chemical vapor deposition method, with varying the methane (CH 4 ) concentration at the stage of bias-enhanced nucleation. It is found from the field-emission scanning electron microscope spectra that nanocrystalline diamond nuclei are formed at a relatively high methane concentration, causing a secondary nucleation at the accompanying growth step. The RMS value of surface roughness for grown films, which is estimated from the atomic force microscope images, monotonically decreases from 165 to 53 nm with methane concentration. It is also observed that the surface roughness is closely related to the nucleation density. In addition, employing a two-step growth method, which consists of first-growth at 500 W and subsequent second-growth at 800 W, enables the as-grown polycrystalline diamond film to have a smaller roughness of approximately 46 nm. This is believed to be due to the secondary nucleation effect induced during the growth step.

Electron-emission from nano- and micro-crystalline diamond films: the effects of nitrogen and oxygen additives

Abstract Diamond films are grown on Si substrate by microwave plasma CVD using CH 4 +H 2 (for undoped) and additive N 2 (for nitrogen-incorporated) with/without O 2 as precursors. Crystal structures for grown films, such as micro- and nano-crystalline and surface morphologies are characterized in terms of growth condition by Raman and field-emission SEM, respectively. Cathodoluminescence (CL) spectra are monitored to identify the nitrogen-incorporation in grown diamond films. Relative intensity ratios of nitrogen-related band to band-A (denoted by I N / I A ) are also estimated from the CL characteristics and the influence of additive N 2 and O 2 precursors on the I N / I A ratio is analyzed. For all-grown films, electron-emission characteristics are measured, from which threshold fields for the emission are also estimated. Observed emission properties are correlated with crystal structures and morphologies obtained from grown films by considering the structural transformation from micro- to nano-crystalline as well as the nitrogen-induced defect states.

Growth of highly-oriented diamond films on 6H–SiC (0001) and Si (111) substrates and the effect of carburization

Abstract The growth of highly oriented diamond is performed on Si (111) and 6H–SiC (0001) substrates via two-step and three-step processes using microwave plasma CVD. The two-step process involves bias-enhanced nucleation (BEN) and deposition, and the three-step process involves carburization in addition to the two-step process. The diamond films grown on the Si (111) substrate exhibit high quality and desirable (111)-orientation under the carburization condition of 5.3×10 3 Pa in pressure with no bias applied. The mechanism for the formation of conversion layer during the carburization step is investigated on both the substrates through the Raman and X-ray photoelectron spectroscopy (XPS) studies. The results indicate that the carburization mainly composed of β-SiC, which plays a crucial role for the formation of the conversion layer and which eventually promotes the diamond nucleation. It is also suggested that a highly-oriented and high-quality diamond film can be successfully achieved by carburization.

Cathodoluminescence characteristics of polycrystalline diamond films grown by cyclic deposition method

Polycrystalline diamond films were deposited using a cyclic deposition method where the H2 plasma for etching (tE) and the CH4+H2 plasma for growing (tG) are alternately modulated with various modulation ratios (tE/tG). From the measurement of full width at half maximum and ID/IG intensity ratio obtained from the Raman spectra, it was found that diamond defects and non-diamond carbon phases were reduced a little by adopting the cyclic deposition method. From the cathodoluminescence (CL) characteristics measured for deposited films, the nitrogen-related band (centered at approximately 590 nm) as well as the so-called band-A (centered at approximately 430 nm) were observed. As the cyclic ratio tE/tG increased, the relative intensity ratio of band-A to nitrogen-related band (IA/IN) was found to monotonically decrease. In addition, analysis of X-ray diffraction spectra and scanning electron microscope morphologies showed that CL characteristics of deposited diamond films were closely related to their crystal orientations and morphologies.