Papers by Keyword: Carrier Mobility

Abstract: Temperature-dependent Hall effect measurements were conducted to investigate the channel conduction mechanisms of 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). This method allows us to discriminate the impact of the density of mobile (free) carriers in the inversion channels and their net mobility on the performance of SiC MOSFETs. It was found that, while the free carrier ratio of SiC MOSFETs with conventional gate oxides formed by dry oxidation is below 4% at 300 K, increasing the free carrier ratio due to thermal excitation of trapped electrons from SiO₂/SiC interfaces leads to an unusual improvement in the field-effect mobility of SiC MOSFETs at elevated temperatures. Specifically, a significant increase in free carrier density surpasses the mobility degradation caused by phonon scattering for thermally grown SiO₂/SiC interfaces. It was also found that, although nitrogen incorporation in SiO₂/SiC interfaces increases the free carrier ratio typically up to around 30%, introduction of an additional scattering factor associated with interface nitridation compensates for the moderate amount of thermally generated mobile carriers at high temperatures, indicating a fundamental drawback of nitridation of SiO₂/SiC interfaces. On the basis of these findings, we discuss the channel conduction mechanisms of SiC MOSFETs.

Abstract: Photoresistors based on amorphous Si-Sn thin films (270-285 nm) with different concentration of tin were studied by spectral measurements of photoconductivity at room temperature and by low-temperature measurements of the Hall effect . Electrical contacts to the Si-Sn film was formed by aluminum layer deposition. When the Al contact is illuminated, the spectral sensitivity of the photoresistor with Sn consentration of 19% extends to 2060 nm due to Schottky barrier influence. It was proved that the Si-Sn alloy film provides photoresponse with cut-off energy of 0.98 eV that is close to the indirect band gap in the Si-Sn film. Three deep acceptor levels with activation energies of 90, 114, and 173 meV were found in the Si-Sn thin film (20% Sn) in the temperature range of 50 – 300 K. Sequential activation of the deep levels and their competition leads to a nonmonotonic change of the Si-Sn film conductivity (0.025 - 5.0 (Ω×cm)^-1) and mobility of holes (100 – 500 cm²/(V·s)). The transition to the intrinsic conductivity region of the amorphous Si-Sn film was not observed up to room temperature.

Abstract: The Monte Carlo approach is used to calculate carrier mobility in molecular copper phthalocyanine (CuPc) with applied electric field in the range of 0.5 to 20 × 10³ kV/cm. Density functional theory (DFT) is employed to derive the molecular interaction between neighboring molecules with various applied electric fields. The result of DFT calculation to evaluate transfer integral that used to calculate hopping rate in the range of applied electric fields. The charge transfer rate between adjacent molecules can be estimated by using the Marcus–Levich–Jortner (MLJ) formalism. The charge is assumed to be localized on the donor and then transferred to the acceptor. Tunneling is modeled by including selected vibration modes at the quantum mechanical level. The result of hopping rate is in ordered of 10¹⁵ s^-1 for hole hopping in direction of applied electric field on the contrary hopping rate in ordered of 10¹⁴ s^-1. The result of mobility can be calculated in range of 0.44 - 10.0 cm²/Vs decrease as a function of applied electric field that calculated by simple hopping model.

Abstract: Heavily doped layers were formed in 4H-SiC device epitaxial structures comprised of moderately doped n layer (channel) and heavily doped p⁺ layer (gate). The n⁺ regions were formed by local ion implantation of nitrogen followed by post-implantation annealing with graphite capping layer. It was shown that annealing at 1700 °C is required for complete activation of implanted impurities. The post-implantation anneals were found to have no significant effect on the moderately nitrogen doped channel layer. On the other hand it resulted in noticeable deterioration of electrical propertied of heavily doped epitaxial p⁺ layers leading to the increase of contact resistivity which has to be taken into account in design and processing of SiC devices.

Abstract: With the breakthrough of mobility in quantum dot electric field transistors (Q-EFTs), the potential application in these functional devices has revealed and been paid more attentions, due to flexibility in design, low cost, facility for processing and large area. One of the most important applications of FETs is the photoconductive detector. However, these functional FETs have less been reported. In this work, colloidal PbS Q-FETs were successfully fabricated by reasonable structure design and layer-by-layer depositon technique PbS quantum-dots. The bipolar property was demonstrated by the output and transfer characteristics, as devices work in I and III quadrants simultaneously. The mobilities of electron and hole are 0.16 cm²/(V⋅s) and 0.28 cm²/(V⋅s), respectively. Q-FETs work as photoconductive detectors at both positive and negative gate bias voltages. Under constant gate bias, photocurrent increase exponentially with the intensity of light. The responding region consisted with the absorption range of PbS quantum dots. A linearity was found in drain voltage and incidence of laser power, the ratio was attributing to 0.0019 (μW⋅V)^-1.

Abstract: The influence of GBs contained in the channel of MOS-FETs - fabricated in thin SOI layers - is demonstrated. The drain current measured at room temperature increases about 50 times for nFETs and about 10 times for pFETs, respectively, as compared to reference devices. The observations might be interpreted as a strong increase of the mobility of charge carriers. Moreover, the observed stepwise changes of the drain current at 5 K may point to Coulomb blockades.

Abstract: The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions.

Abstract: 3C-SiC have been epitaxially grown through vapour phase expitaxy under a different grow conditions. Key electrical properties of these SiC layers have been characterised by fabrication and measurement of metal-SiC-metal devices. The electrical properties of SiC grown at different conditions have been analysed based on their structural and crystalline quality.

Abstract: The desorption process for ambient atmosphere on electrical transport properties of bilayer graphene FET grown by CVD methods on SiO2/Si substrate was investigated in room temperature. With increasing the vacuum time of the device underwent, we found that the voltage of Dirac point decreased, the mobility of hole (electron) increased and the charged impurity density decreased. The results suggest that the atmospheric adsorbates (mainly oxygen and water molecules) are strongly influence the electrical transport properties of graphene FET.

Abstract: A triphenylene-based discotic compound 2,3,6,7,10,11-hexaheptylcar-boxyltriphenylene was synthesized. The structure of the compound was determined by ¹HNMR and FT-IR spectrum and its mesomorphism was clarified by DSC and POM methods. The carrier mobility was measured using time-of-flight method and has a value of 1.2 × 10^-1cm²V^-1s^-1.