Moon-Ho Lee

Relationship between Surface Roughness of Indium Tin Oxide and Leakage Current of Organic Light-Emitting Diode

The relationship between surface roughness of indium tin oxide (ITO) and leakage current of organic light-emitting diode (OLED) was investigated. The surface roughness of ITO was measured with atomic force microscope (AFM) before the device fabrication by using these substrates, OLEDs were fabricated and their electrical properties were measured. leakage current of OLEDs depending on the surface roughness of the substrate. In this letter, we report that leakage currents of OLEDs are highly dependent on the peak-to-valley roughness (Rpv) of ITO.

Thermal properties of a pyroelectric-ceramic infrared detector with metallic intermediate layer

infrared thermal detectors were prepared with pyroelectric PSN-PT-PZ (1/47/52) ceramics, where a signal electrode had a structure Au/metallic buffer/Pb(Zr,Ti)O3 ceramic. The effect of a metallic buffer layer on the voltage responsivity was investigated with a response to a step signal, made by a dynamic pyroelectric measurement. A pyroelectric ceramic wafer was prepared by a mixed-oxide technique. The Au layer (thickness 50 nm) and the metallic buffers (thickness 0 to 20 nm) of Cr, NiCr (80 : 20), and Ti were prepared by dc magnetron sputtering. In order to improve the light absorptivity, Au black was coated on the Au signal electrode by thermal evaporation. A detector without a buffer layer showed a noisy and fluctuating output signal. Among the three kinds of buffer materials, NiCr (80 : 20) and Ti adhered well with ceramics and showed good electrical and thermal contacts, whereas Cr resulted in bad contacts. Considering the output voltage and thermal properties, the optimum thickness of the buffer layer was about 15 to 20 nm, and sensors with a Ti buffer 15 to 20 nm in thickness showed good detectivity. Thus, the stability and reliability of the infrared thermal sensors could be improved by using an appropriate buffer layer.

Piezoelectric Properties of (Ba, Ca)TiO3 Doped with SiO2, Al2O3 and MnO2

The dielectric and piezoelectric properties of Ba0.917Ca0.083TiO3 ceramics doped with SiO2, Al2O3 and MnO2 were studied as a function of SiO2 and/or Al2O3 contents, and Mn content. The addition of SiO2 and/or Al2O3 deteriorates the piezoelectric coupling factor (kp) and the mechanical quality factor (Qm). On the other hand, the addition of MnO2 improves the deteriorated piezoelectric properties (i.e., kp and Qm) of the (Ba, Ca) TiO3 ceramics doped with SiO2 and/or Al2O3. A high kp of over 40% was found at the doping of 3.0–6.0% MnO2. From the analysis of EDS results, it is supposed that the increase of kp and Qm is related with the distribution of Mn in grain interior or boundary.

Characteristic of thin-film NTC thermal sensors

The characteristics of thin-film negative temperature coefficient of resistance (NTC) thermal sensors fabricated by micromachining technology were studied as a function of the thickness of the membrane. The overall structure of the thermal sensor has the form of Au/Ti/NTC/SiO/sub x//(100)Si. An NTC film of Mn/sub 1.5/CoNi/sub 0.5/O/sub 4/ with 0.5 /spl mu/m thickness was deposited on the SiO/sub x/ layer (1.2 /spl mu/m) by PLD (pulsed laser deposition) and annealed at 600-800/spl deg/C in air for 1 hr. Au (200 nm)/Ti (100 nm) electrode was coated on the NTC film by DC sputtering. From the results of microstructure, X-ray and NTC analysis, post-annealed NTC films at 700/spl deg/C for 1 hr showed the best characteristics as an NTC thermal sensing film. In order to reduce the thermal mass and thermal time constant of sensor, the sensing element was built-up on a thin membrane with the thickness of 20-65 /spl mu/m. Sensors with a thin sensing membrane show good detection characteristics.

Residual stresses in the electrode of Pt/Ti for the thermal detector with thin-film structure

Residual stress of Pt/Ti thin film electrode was investigated as a function of substrate temperature by using of XRD and SEM. In order to develop the residual stress, stress sensitive structures of Pt/Ti thin film on SiO/sub 2//Si(substrate) were fabricated. Thermally induced residual stress, which was originated in the different thermal expansion coefficients between film and substrate, was led to the dimensional change of overall structure. XRD results confirmed the existence of internal stresses on Pt/Ti thin film.

Effect of buffer layer on the voltage responsivity of the pyroelectric thermal sensors prepared with PZT ceramics

The pyroelectric thermal detectors were prepared with lead zirconate titanate (PZT) ceramics, where a signal electrode had a structure of Au/metallic buffer/(PZT ceramic). The effect of buffer layer on the voltage responsivity was investigated with a response to step signal, taken by dynamic pyroelectric measurement. Pyroelectric ceramic wafer was prepared by mixed oxide technique. Au layer (thickness: 50 nm) and metallic buffers (thickness: 0 - 20 nm) of Cr, NiCr (80/20), and Ti were prepared by dc magnetron sputtering. In order to improve the light absorptivity, an Au-black was coated on Au signal electrode by thermal evaporation. At steady state, the output voltage (Vo) was decreased with increasing chopping frequency in the range of 1 - 100 Hz. A sensor without buffer showed the severe time-drift and instability in the output signal. However, the sensors with buffer layer showed the stable outputs. For step radiations, rising time (tp), peak voltage (Vp), and initial slope (k) of the output voltage were dependent upon the thickness and materials of buffer layer. The mechanical and electrical contacts between Au electrode and PZT ceramics were improved by inserting the metallic buffer layer. Considering the characteristics of the output voltage, the optimum thickness of buffer layer was about 15 - 20 nm, and the sensors with Ti buffer of 15 - 20 nm in thickness showed the good detectivity. Therefore, the stability and reliability of the thermal sensors could be improved by use of appropriate buffer layer.

Detectivity of thin-film NTC infrared sensors

Characteristics of thin-film NTC infrared sensors fabricated by micromaching technology were studied as a function of the thickness of membrane. The overall-structure of thermal sensor has a form of Au/Ti/NTC/SiOx/(100)Si. NTC film of Mn1.5CoNi0.5O3 with 0.5 micrometers in thickness was deposited on SiOx layer (1.2 mm) by PLD (pulsed laser deposition) and annealed at 600-800 degree(s)C in air for 1h. Au (400nm)/Ti (100nm) electrode was coated on NTC film by dc sputtering. By the results of microstructural, X-ray and NTC analysis, post-annealed NTC films at 700 degree(s)C for 1h showed the best characteristics as NTC thermal sensing film. In order to reduce the thermal mass and thermal time constant of sensor, the sensing element was built-up on a thin membrane with the thickness of 20-65mm. Sensors with thin sensing membrane show the good detecting characteristics.

Detectivities of thin film NTC infrared sensors

Detecting characteristics of thin-film NTC infrared sensors fabricated by micromachining technology were studied. In order to reduce the thermal mass and increase the responsivity, the sensing element was built-up on a thin membrane. The overall structure of detector had a form of Au/Ti/NTC/SiO/sub x//[100]Si. Detecting characteristics of NTC sensor were monitored by the dynamic method. B, TCR and R/sub 25/spl deg/C/ of sensors are 3,249K, 3.7%//spl deg/C and 4M/spl Omega/, which are suitable for bolometer applications. And, the rise time is 80msec, so it is appropriate for thermal sensors.

Spike-like noises of pyroelectric thermal detector

An analysis of spike-like noise for the thermal detector prepared with Pb(Zr,Ti)O3-Pb(Sb0.5Nb0.5) (PZT- PSN) pyroelectric ceramics as sensing element was conducted by measuring its oven noise as a function of the JFET characteristics, gate resistance, low-temperature heat treatment, chemical composition and grain size of pyroelectric ceramic. Pyroelectric wafers were prepared by mixed oxide technique, and thermal sensor fabricated with a PZT-PSN ceramic wafer, JFET, chip-type gate resistor and TO- 5 package with AR coated Si-window. White noises depended on the characteristics of JFET, gate resistance and chemical composition of puyroelectric materials. Output spikes originated in JFET were removed by the pyroelectric sensing wafer with high capacitance. Pyroelectric element generated the temperature-induced transient noise during cooling, which were remarkably reduced in their amplitude and frequency by heat-treating at low temperature and by decreasing the grain size of pyroelectric ceramic. The spike-like transient noise is caused by the twinning and domain switching to reduce the thermally induced elastic energy within the sensing element, originated in the different thermal expansion between pyroelectric ceramic and alumina substrate.