To understand the contact mechanism from electrical properties of the ZnS TFTs, ZnS was fabricated on SiOC as a gate insulator on a Si substrate. Ohmic contact without a potential barrier increased the leakage current, but Schottky contact decreased the leakage current because of a Schottky barrier (SB). The ZnS TFTs prepared on SiOC with a Schottky contact improved the stability with respect to the reduction of drain voltages. The structural matching between ZnS and SiOC increased the height of SB such as ZnS annealed at 200°C, which made ZnS become an amorphous structure. ZnS/SiOC films with a low SB increased the capacitance and leakage current. The crystallinity orientation of ZnS localized defect states and the drift current owing to the impurity charge carriers caused the leakage current through low SB near zero voltages. But the increment of diffusion currents in a depletion layer increased the SB and then decreased the leakage current. So the electrical properties of devices were improved by a tunneling effect of diffusion currents.
Thin films of wide band gap II–VI compounds, such as CdS, CdSe, and ZnS, have received considerable attention as photoelectronic materials. ZnS was used in a buffer layer for the fabrication of solar cells. Its advantages include large energy band gap, nontoxic environmental materials, and superior optical properties that make ZnS suitable for nanoscale devices [
In this work, the ZnS thin film transistor with a gate insulator SiOC was fabricated to research the Ohmic and Schottky contacts depending on annealing temperatures. In order to understand the role of conduction mechanism on the thin film transistor with ZnS channel, the capacitance, leakage current,
To obtain electrical characteristics and contacts of TFTs, the ZnS/SiOC TFTs were fabricated. SiOC as a gate insulator was prepared by the RF sputtering system using the target of SiOC, which was prepared on p-type Si substrates at room temperature and RF power of 250 W for 20 minutes. ZnS was also grown on SiOC/Si by the RF sputtering system with 70 W for 10 minutes. ZnS/SiOC films were annealed at 100, 200, and 300°C to observe the Ohmic/Schottky contacts. The target to substrate distance was kept at 100 mm and the base pressure was 4.5 × 10−5 Pa and the working pressure of the chamber with oxygen gas was 1.2~1.4 × 10−3 Torr. The oxygen (99.9999%) was controlled by mass flow controller (MFC) for 20 min. For the analysis of electric properties, the aluminum was evaporated using the mask pattern on the surface of the specimen, where the sputter-deposited aluminum with an area of
The depletion layer, which is related to a height of Schottky barrier (SB), is an important factor to define Schottky and Ohmic contacts [
The crystalline orientation of the ZnS was determined at about 29.3° and 33.6° using XRD patterns with annealing temperatures as shown in Figure
XRD patterns of ZnS with annealing temperatures near 2 theta of (a) 29.3° and (b) 33.6°.
Figure
Comparison of PL spectra between ZnS, SiOC, and ZnS/SiOC.
Figure
PL spectra with annealing temperatures, (a) ZnS and (b) ZnS/SiOC.
Figure
Electrical properties of ZnS TFT on SiOC/Si substrate, (a)
Figure
To understand the correlation between diffusion currents and
Log plotted
Figure
Figure
Electrical properties of structure with Al/ZnS/SiOC/Si substrates to research a contact mechanism, (a) current of
Figure
Current-voltage of Al/ZnS/SiOC/Si structure, (a) Ohmic contact and (b) Schottky contact of sample annealed at 200°C in the region of
This research included the electrical properties depending on the contact mechanism of ZnS/SiOC TFT devices. The electrical characteristics of ZnS/SiOC depended on the annealing temperatures. The transfer characteristics of ZnS/SiOC TFT were shown as the ambipolar properties with respect to high Schottky barrier, which was made from the reduction of polarities in SiOCs as a gate insulator. The structural matching between ZnS and SiOC increased the height of SB for a band to band tunneling phenomenon and improved the performance of TFTs. The induction of a depletion layer also caused the decrement of leakage currents by the increment of SB, which was made from the diffusion current fabricated by an electron hole combination due to the difference between carrier concentrations. The diffusion current in a depletion layer and drift current in a channel are two kinds of conduction mechanism in semiconductor devices. The diffusion current for a band to band tunneling was proportional to the height of SB, and the leakage current decreased with increasing the SB. Finally, the electrical performance of devices was improved.
The author declares that there is no conflict of interests regarding the publication of this paper.