No abstract

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au ͑30 nm/100 nm/30 nm/30 nm͒ multilayer Ohmic contacts to n-GaN ͑doping level 5ϫ10 17 cm Ϫ3) were studied. The lowest contact resistivity derived from the annealed contact was S ϭ3.0ϫ10 Ϫ6 ⍀ cm 2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti-Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to the formation of Ti-Al alloys with the remaining Al content. Consequently, both the Al and the Ti ͑from the second Ti layer͒ contents were almost fully consumed, and none of them were left to appear on the contact surface to form oxides.

The authors report transport property measurements of individual GaN nanowire field effect transistors and the correlation of the electron mobilities with the existence of grain boundaries in these nanowires. Room temperature field effect electron mobilities as high as 319cm2V−1s−1 were obtained for the 200nm diameter nanowires. Mobilities calculated from these reliable nanowire field effect transistors indicated that the surface scattering plays a dominant role in smaller diameter nanowires, whereas for intermediate diameter devices transport is dominated by grain boundary scattering. Reduction of the mobility with decreasing diameter of nanowires can be explained using “continuous surface” model.

We have studied temperature-dependent (77-300 K) electrical characteristics and low-frequency noise (LFN) in chemical vapor deposited (CVD) single-layer molybdenum disulfide (MoS2) based back-gated field-effect transistors (FETs). Electrical characterization and LFN measurements were conducted on MoS2 FETs with Al2O3 top-surface passivation. We also studied the effect of top-surface passivation etching on the electrical characteristics of the device. Significant decrease in channel current and transconductance was observed in these devices after the Al2O3 passivation etching. For passivated devices, the two-terminal resistance variation with temperature showed a good fit to the activation energy model, whereas for the etched devices the trend indicated a hopping transport mechanism. A significant increase in the normalized drain current noise power spectral density (PSD) was observed after the etching of the top passivation layer. The observed channel current noise was explained using a standard unified model incorporating carrier number fluctuation and correlated surface mobility fluctuation mechanisms. Detailed analysis of the gate-referred noise voltage PSD indicated the presence of different trapping states in passivated devices when compared to the etched devices. Etched devices showed weak temperature dependence of the channel current noise, whereas passivated devices exhibited near-linear temperature dependence.