Here, we report the synthesis of a vertical MoSe 2 /WSe 2 p-n heterostructure using a sputtering-CVD method. Unlike the conventional CVD method, this method produced a continuous MoSe 2 /WSe 2 p-n heterostructure. WSe 2 and MoSe 2 back-gated field effect transistors (FETs) exhibited good gate modulation behavior, and high hole and electron mobilities of $2.2 and $15.1 cm 2 V À1 s À1 , respectively.The fabricated vertical MoSe 2 /WSe 2 p-n diode showed rectifying I-V behavior with back-gate tunability.The rectification ratio of the diode was increased with increasing gate voltage, and was increased from $18 to $1600 as the gate bias increased from À40 V to +40 V. This is attributed to the fact that the barrier height between p-WSe 2 and n-MoSe 2 is modulated due to the back-gate bias. The rectification ratio is higher than the previously reported values for the TMDC p-n heterostructure grown by CVD.

Herein, the fabrication of a novel highly sensitive and fast hydrogen (H2) gas sensor, based on the Ta2O5 Schottky diode, is described. First, Ta2O5 thin films are deposited on silicon carbide (SiC) and silicon (Si) substrates via a radio frequency (RF) sputtering method. Then, Pd and Ni are respectively deposited on the front and back of the device. The deposited Pd serves as a H2 catalyst, while the Ni functions as an Ohmic contact. The devices are then tested under various concentrations of H2 gas at operating temperatures of 300, 500, and 700 °C. The results indicate that the Pd/Ta2O5 Schottky diode on the SiC substrate exhibits larger concentration and temperature sensitivities than those of the device based on the Si substrate. In addition, the optimum operating temperature of the Pd/Ta2O5 Schottky diode for use in H2 sensing is shown to be about 300 °C. At this optimum temperature, the dynamic responses of the sensors towards various concentrations of H2 gas are then examined under a constant bias current of 1 mA. The results indicate a fast rise time of 7.1 s, and a decay of 18 s, for the sensor based on the SiC substrate.

The key process steps for growing high-quality Si-based epitaxial films via reduced pressure chemical vapor deposition (RPCVD) are investigated herein. The quality of the epitaxial films is largely affected by the following steps in the epitaxy process: ex-situ cleaning, in-situ bake, and loading conditions such as the temperature and gaseous environment. With respect to ex-situ cleaning, dry cleaning is found to be more effective than wet cleaning in 1:200 dilute hydrofluoric acid (DHF), while wet cleaning in 1:30 DHF is the least effective. However, the best results of all are obtained via a combination of wet and dry cleaning. With respect to in-situ hydrogen bake in the presence of H2 gas, the level of impurities is gradually decreased as the temperature increases from 700 °C to a maximum of 850 °C, at which no peaks of O and F are observed. Further, the addition of a hydrogen chloride (HCl) bake step after the H2 bake results in effective in-situ bake even at temperatures as low as 700 °C. In addition, the effects of temperature and environment (vacuum or gas) at the time of loading the wafers into the process chamber are compared. Better quality epitaxial films are obtained when the samples are loaded into the process chamber at low temperature in a gaseous environment. These results indicate that the epitaxial conditions must be carefully tuned and controlled in order to achieve high-quality epitaxial growth.