Moon Suk Choi

Structural, optical and chemical analysis of zinc sulfide thin film deposited by RF-mganetron sputtering and post deposition annealing

Zinc sulfide (ZnS) thin films were deposited by radio-frequency (RF) magnetron sputtering. The effects of the process parameters such as deposition time and RF-power, as well as of post deposition annealing under oxygen containing atmospheres, on the material properties of ZnS films have been investigated. X-ray diffraction analysis reveals out that the as-deposited ZnS films preferred (002) hexagonal wurtzite and (111) cubic zinc blend (111) at 28.60°, while a thicker ZnS film has additional hexagonal wurtzite (100), (110), and (200) planes coexisting with the preferred oriented-planes, suggesting that the thickness is dependent on the growth of ZnS. After annealing, ZnO phases were detected, indicating island-like grain growth on the surface of the ZnS film. By increasing the deposition time and the RF power, the optical band gap energy (Eg) of the ZnS film changes from 4.13 to 3.87 eV, indicating the presence of lower Eg with thicker ZnS film. The lower Eg (∼3.27 eV) value of the annealed films is attributed to the ZnO transition. Unlike bulk ZnS material (Zn/S∼1.08), deposited ZnS thin film has Zn-rich and S-deficient composition (Zn/S∼1.28). However, the Zn/S ratio is closer to the ideal value when there is a longer deposition time or higher RF-power.

Loss and Heat Transfer Analysis for Reliability in High Speed and Low Torque Surface Mounted PM Synchronous Motors

It is essential to predict the coil temperature under over load and over speed conditions for reliability in high speed low torque surface mounted PM synchronous motors(SPM). In the present study, the losses and coil temperature are measured under rated condition and calculated under over speed and over load conditions in the three different motors with 35PN440, 25PN250 and 15HTH1000. The heat transfer modeling has been performed based on acquired losses and temperature. The difference of coil temperature between heat transfer modeling and experiment is less than 6.4% under no load, over speed and over load conditions. Subsequently, the coil temperature of the motor with 15HTH1000 is 84.4% of the coil temperature of the motor with 35PN440 when speed is 0.9 and load is 3.0. The output of motor with 15HTH1000 is 85.2% greater than the output of the motor with 35PN440 when the dimensionless coil temperature is 1.0.

A Study of Sputtered TiN Gate Electrode Etching with Various Wet Chemicals and Post Etch Annealing for Complementary Metal–Oxide–Semiconductor Device Integration Applications

Wet chemicals for etching sputtered TiN metal gate and post etch annealing on HfO2 and HfSiON gate dielectrics were studied with metal–oxide–semiconductor devices. Various wet solutions such as SC1 (NH4/H2O2/H2O= 1:2:5), SPM (H2SO4/H2O2= 10:1), and H2O2 were employed to etch the sputtered TiN. Difference in equivalent oxide thickness (EOT) is negligible among etchants while the lowest leakage current density (Jg) is attained with only SPM solution. Even though SPM treatment shows relative poor surface morphologies compared to H2O2 process, difference in Jg is mainly affected by the amount of absorbed Ti into high-k gate dielectrics during wet etch process. Lower Jg using SPM is attributable to the reduced amount of Ti-adsorption due to additional H2SO4 acid in wet chemical solution, which is confirmed by total reflection X-ray fluorescence. Post etch annealing on high-k layer improves film qualities such as suppressed defects – less frequency dependence – and lowers Jg further while EOT is slightly increased by about 0.2 nm due to SiO2 interfacial regrowth. HfSiON gate dielectric shows stronger immunity against TiN wet etch compared with HfO2. Thus, appropriate etchant and post annealing for the selective TiN etching are carefully considered to suppress defects and Jg for attaining complementary metal–oxide–semiconductor (CMOS) device.

Performance Loss & Heat Transfer Characteristics of Synchronous Motors under Various Driving Conditions

Core loss has a major effect on heat generation in synchronous motors with surface-mounted permanent magnets (SPMs). It is essential to perform heat transfer analysis considering core loss in SPM because core loss is seriously affected by torque and speed of motors. In the present study, mechanical loss, core loss and coil loss are evaluated by measuring input and output energies under various driving conditions. For a better understanding heat transfer paths in synchronous motors, we developed a lumped thermal system analysis model. Subsequently, heat transfer analysis has been performed based on acquired energy loss, temperature data and thermal resistance with three types of SPM. It is shown that the torque constants decrease by Max. 10% as speed increase. At the rated torque, the core loss is Max. 10.9 times greater than the coil loss and the hysteresis loss of magnets is dominant in total loss.

Influence of Different Annealing Ambients on the Properties of Zinc Sulfide Prepared by Atomic Layer Deposition

The effects of different post annealing ambients (vacuum, O2, and H2S gases) on the chemical, structural, and optical properties of zinc sulfide (ZnS) thin films prepared by atomic layer deposition (ALD) were investigated. Diethylzinc [DEZ, Zn(C2H5)2] and H2S gas were used as precursor and reactant gas, respectively. Compared to as-deposited 50-nm-thick ZnS film, the optical energy band gap (Eg) of ZnS annealed under vacuum and H2S conditions increased from 3.73 to 3.85 eV, while it decreased down to 3.23 eV for the O2 annealing case. The change in the Eg of the thicker ZnS is similar to that of the thinner ZnS case. This behavior is related to the change in the Zn to S ratio. The vacuum and H2S anneals increases the Zn/S ratio, leading to higher Zn interstitial defects or S vacancy sites in the films. X-ray diffraction analysis reveals that ZnS thin film has a preferred orientation of hexagonal wurtizte (002) and cubic zinc blend (111) at ~28.2°, and its grain size changes in a range from 18.79 to 28.14 nm after annealing. However, for O2 annealing, the patterns of both the newly formed ZnO phase and the reduced ZnS phase appear at 34.04°. This result suggests that change in the composition and crystal structure during the process significantly affects the optical properties of ZnS thin film, which should be taken into consideration in searching for an alternative buffer layer for Cu2InGaSe(S)4 (CIGS) thin film solar cell systems.