Shaharin Fadzli Abd Rahman

Room temperature nonlinear operation of a graphene-based three-branch nanojunction device with chemical doping

A chemically doped graphene-based three-branch nanojunction device is fabricated on a SiO2/p-Si substrate, and its nonlinear operation is characterized at room temperature (RT). By polyethyleneimine doping, the fabricated device shows improved field effect mobility of 14 800 and 16 100 cm2/Vs for electron and holes, respectively. The device clearly exhibits nonlinearity in voltage transfer curves at RT. The curvature of the transfer curve can be controlled by using the back gate voltage, and its polarity abruptly switches near the Dirac point because of the carrier type change. The observed behaviour can be quantitatively explained in terms of the difference in the amounts of gate-induced carriers in the two input branches.

Open-Gated pH Sensor Fabricated on an Undoped-AlGaN/GaN HEMT Structure

The sensing responses in aqueous solution of an open-gated pH sensor fabricated on an AlGaN/GaN high-electron-mobility-transistor (HEMT) structure are investigated. Under air-exposed ambient conditions, the open-gated undoped AlGaN/GaN HEMT only shows the presence of a linear current region. This seems to show that very low Fermi level pinning by surface states exists in the undoped AlGaN/GaN sample. In aqueous solution, typical current-voltage (I-V) characteristics with reasonably good gate controllability are observed, showing that the potential of the AlGaN surface at the open-gated area is effectively controlled via aqueous solution by the Ag/AgCl gate electrode. The open-gated undoped AlGaN/GaN HEMT structure is capable of distinguishing pH level in aqueous electrolytes and exhibits linear sensitivity, where high sensitivity of 1.9 mA/pH or 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V is obtained. Due to the large leakage current where it increases with the negative gate voltage, Nernstian like sensitivity cannot be determined as commonly reported in the literature. This large leakage current may be caused by the technical factors rather than any characteristics of the devices. Surprisingly, although there are some imperfections in the device preparation and measurement, the fabricated devices work very well in distinguishing the pH levels. Suppression of current leakage by improving the device preparation is likely needed to improve the device performance. The fabricated device is expected to be suitable for pH sensing applications.

Boolean Logic Gates Utilizing GaAs Three-Branch Nanowire Junctions Controlled by Schottky Wrap Gates

A GaAs-based three-branch nanowire junction (TBJ) with Schottky wrap gates (WPGs) is investigated to realize novel Boolean logic gates. The WPG-controlled TBJ shows a bell-shaped voltage input–output curve and is controlled by gate voltage on the WPGs. The observed characteristics are explained using a simple equivalent circuit model. AND gate operation is realized in the WPG-controlled TBJ and its output voltage swing is controlled using WPGs. It can also operate as a NOT gate by changing the measurement circuit. A NAND gate is fabricated by integrating two WPG-controlled TBJs, and correct operation with a voltage transfer gain of 2.2 is realized.

Dual-Functional On-Chip AlGaAs/GaAs Schottky Diode for RF Power Detection and Low-Power Rectenna Applications

A Schottky diode has been designed and fabricated on an n-AlGaAs/GaAs high-electron-mobility-transistor (HEMT) structure. Current-voltage (I–V) measurements show good device rectification, with a Schottky barrier height of 0.4349 eV for Ni/Au metallization. The differences between the Schottky barrier height and the theoretical value (1.443 eV) are due to the fabrication process and smaller contact area. The RF signals up to 1 GHz are rectified well by the fabricated Schottky diode and a stable DC output voltage is obtained. The increment ratio of output voltage vs input power is 0.2 V/dBm for all tested frequencies, which is considered good enough for RF power detection. Power conversion efficiency up to 50% is obtained at frequency of 1 GHz and input power of 20 dBm with series connection between diode and load, which also shows the device’s good potential as a rectenna device with further improvement. The fabricated n-AlGaAs/GaAs Schottky diode thus provides a conduit for breakthrough designs for RF power detectors, as well as ultra-low power on-chip rectenna device technology to be integrated in nanosystems.

Graphene as a Buffer Layer for Silicon Carbide-on-Insulator Structures

We report an innovative technique for growing the silicon carbide-on-insulator (SiCOI) structure by utilizing polycrystalline single layer graphene (SLG) as a buffer layer. The epitaxial growth was carried out using a hot-mesh chemical vapor deposition (HM-CVD) technique. Cubic SiC (3C-SiC) thin film in (111) domain was realized at relatively low substrate temperature of 750 °C. 3C-SiC energy bandgap of 2.2 eV was confirmed. The Si-O absorption band observed in the grown film can be caused by the out-diffusion of the oxygen atom from SiO2 substrate or oxygen doping during the cleaning process. Further experimental works by optimizing the cleaning process, growth parameters of the present growth method, or by using other growth methods, as well, are expected to realize a high quality SiCOI structure, thereby opening up the way for a breakthrough in the development of advanced ULSIs with multifunctionalities.

Identification of Graphene Layer Numbers from Color Combination Contrast Image for Wide-Area Characterization

Identification of the number of graphene layers using an optical microscope images taken at various magnifications is investigated from the viewpoint of simple wide-area inspection. For graphene on 300-nm-thick SiO2, combination of red and green color contrast gives more accurate contrast value and provides better contrast even at the low magnification as compared with the single color channel contrast. The color combination with suitable weighting factors taking account of light wavelength and intensity dependences of the system response results in the contrast that agrees well with the theoretical values from Fresnels law. Simple image processing is also investigated to improve the signal-to-noise ratio (SNR) of the image. Median filtering improves the SNR of the image having high pixel density, whereas dithering is effective for the low magnification image having block noise due to low pixel density.

Gateless-FET undoped AlGaN/GaN HEMT structure for liquid-phase sensor

A gateless field-effect-transistor (FET) device fabricated on undoped AlGaN/GaN high-electron-mobility-transistor (HEMT) structure is investigated as a liquid-phase sensor. Good gate controllability for typical current-voltage (I-V) characteristics of FET is observed. This result shows that an undoped-AlGaN surface at the open-gate area is effectively controlled by the isolated gate voltage via chemical solution. Stable pH sensing operation in aqueous solution is observed where this device exhibits a high linear sensitivity of 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V. Due to the occurrence of large leakage current, the Nernstians like sensitivity is not observed. It is also found that the device is sensitive to changes in electrostatic boundary conditions of the polar liquids. This indicates that the change in dipole moment in each liquid causes the potential change at AlGaN surface.

RF-DC power conversion of Schottky diode fabricated on AlGaAs/GaAs heterostructure for on-chip rectenna device application in nanosystem

The Schottky diodes enjoined with coplanar waveguides are investigated for applications in on-chip rectenna device application without insertion of a matching circuit. The design, fabrication, DC characteristics and RF-to-DC conversion of the AlGaAs/GaAs HEMT Schottky diode is presented. The RF signals are well converted by the fabricated Schottky diodes with cut-off frequency up to 20 GHz estimated in direct injection experiments. The outcomes of these results provide conduit for breakthrough designs for ultra-low power on-chip rectenna device technology to be integrated in nanosystems.

RF characterization of planar dipole antenna for on-chip integration with GaAs-based schottky diode

The design and RF characteristics of planar dipole antenna facilitated with coplanar waveguide structure was presented. The dipole antennas were fabricated on semi-insulated GaAs substrates by using standard photolithography and lift-off process. As expected, it can be seen that the fundamental resonant frequency shift to higher frequency when the length of antenna decreases. Interestingly, the resonant frequencies of antenna are almost unchanged with the variation of antenna width and metal thickness. The width of dipole antenna and metal thickness only has an effect on the magnitude of return loss where the magnitude increases to more negative value with the increase of width and decrease of metal thickness. One of the most promising applications of our proposed dipole antenna is the capability to be integrated directly with AlGaAs/GaAs Schottky diode without any insertion of matching circuit between them.

Fabrication and Characterization of Planar Dipole Antenna Integrated with GaAs Based-Schottky Diode for On-chip Electronic Device Application

The design and RF characteristics of planar dipole antennas facilitated with coplanar waveguide (CPW) structure on semi-insulated GaAs are performed and confirmed to work in super high frequency (SHF) range. As expected, the fundamental resonant frequency shifts to higher frequency when the length of antenna decreases. Interestingly, the resonant frequencies of antenna are almost unchanged with the variation of antenna width and metal thickness. It is shown experimentally that return loss down to −54 dB with a metal thickness of 50 nm is obtainable. Preliminary investigation on design, fabrication, and DC and RF characteristics of the integrated device (planar dipole antenna + Schottky diode) on AlGaAs/GaAs HEMT structure is presented. From the preliminary direct irradiation experiments using the integrated device, the Schottky diode is not turned on due to weak reception of RF signal by dipole antenna. Further extensive considerations on the polarization of irradiation etc. need to be carried out in order to improve the signal reception. These preliminary results provide a new breakthrough for on-chip electronic device application in nanosystems.