Katrina Morgan

Switching kinetics of SiC resistive memory for harsh environments

Cu/a-SiC/Au resistive memory cells are measured using voltage pulses and exhibit the highest ROFF/RON ratio recorded for any resistive memory. The switching kinetics are investigated and fitted to a numerical model, using thermal conductivity and resistivity properties of the dielectric. The SET mechanism of the Cu/a-SiC/Au memory cells is found to be due to ionic motion without joule heating contributions, whereas the RESET mechanism is found to be due to thermally assisted ionic motion. The conductive filament diameter is extracted to be around 4nm. The high thermal conductivity and resistivity for the Cu/a-SiC/Au memory cells result in slow switching but with high thermal reliability and stability, showing potential for use in harsh environments. Radiation properties of SiC memory cells are investigated. No change was seen in DC sweep or pulsed switching nor in conductive mechanisms, up to 2Mrad(Si) using 60Co gamma irradiation.

Total ionizing dose response of fluorine implanted silicon-on-insulator buried oxide

A comparison is made of the behavior of silicon on insulator buried oxides and wet thermal oxides before and after fluorine implantation and irradiation. Before irradiation, the electrical characteristics of the thermal oxide and buried oxide are significantly different. The fluorine implantation in the smart-cut® buried oxide results in a large negative threshold shift due to the trapping of positive charges. These charges are associated with positively charged fluorine ions on implantation and are trapped at pre-existing trap sites, particularly at the bonding interface, and at additional defects caused by the ion implantation damage. This shift is absent in the wet thermal oxide. After Co60 irradiation up to 500 Krad(Si), the negative flatband and threshold voltage shift in the fluorine implanted buried oxide is larger than in the unimplanted buried oxide indicating that any potential positive effect of fluorine on the passivation of interface states is more than offset by the additional trapping sites created during implantation. These results demonstrate that in the design of a transistor utmost care must be taken to prevent any fluorine being implanted into the buried oxide.

Compliance-free ZrO2/ZrO2 - x /ZrO2 resistive memory with controllable interfacial multistate switching behaviour

A controllable transformation from interfacial to filamentary switching mode is presented on a ZrO2/ZrO2 − x/ZrO2 tri-layer resistive memory. The two switching modes are investigated with possible switching and transformation mechanisms proposed. Resistivity modulation of the ZrO2 − x layer is proposed to be responsible for the switching in the interfacial switching mode through injecting/retracting of oxygen ions. The switching is compliance-free due to the intrinsic series resistor by the filaments formed in the ZrO2 layers. By tuning the RESET voltages, controllable and stable multistate memory can be achieved which clearly points towards the capability of developing the next-generation multistate high-performance memory.

Total dose hardness of TiN/HfOx/TiN resistive random access memory

Resistive random access memory based on TiN/HfO_x/TiN has been fabricated, with the stoichiometry of the HfOx layer altered through control of atomic layer deposition (ALD) temperature. Sweep and pulsed electrical characteristics were extracted before and after ⁶⁰Co gamma irradiation. Monoclinic HfO_x deposited at 400^°C did not result in resistive switching. Deposition at 300^°C and 350^°C resulted in cubic HfO_x which switched successfully. Both stoichiometric HfO₂ and sub-oxides HfO_2-x result in similar memory characteristics. All devices are shown to be radiation hard up to 10 Mrad(Si), independent of stoichiometry.

The effect of atomic layer deposition temperature on switching properties of HfOx resistive RAM devices

TiN/HfOx/TiN resistive RAM (RRAM) devices have been fabricated where the hafnium oxide layer has been deposited at three different temperatures via atomic layer deposition (ALD). Material characterization shows the structure of the hafnium oxide is converted from cubic to monoclinic for 400 degrees C. Elemental analysis shows that the temperature affects the stoichiometric behavior of hafnium oxide, with a higher oxygen concentration at 350 degrees C and above. The switching behavior differs significantly for each device whereby the 400 degrees C device shows no successful switching, due to the change in structure to monoclinic. The two lower temperatures both show successful bipolar switching which set at negative voltages. The 300 degrees C device has a higher Roff/Ron of 13.9, with superior endurance. The 350 degrees C device has a lower Roff/Ron of 5.5 and shows deterioration in switching properties as the number of cycles are increased. At 300 degrees C, the oxygen hafnium ratio is at a minimum; hence the greatest amount of oxygen vacancies are present, which results in improved switching characteristics. This supports the theory that oxygen vacancies play a key role in the switching mechanism for metal oxide RRAM devices.

Total Ionizing Dose and random dopant fluctuation effects in 65-nm gate length partially depleted Silicon-on-Insulator nMOSFETs

The contribution of random dopant fluctuations in the post-irradiation response of deep sub-micron Silicon-Insulator MOSFETs is examined with the use of TCAD simulation tools. The variation in the off-state leakage current is quantified for different Total Ionizing Dose charge concentrations in the buried oxide and shallow trench isolation. The results show that depending on the charge in the oxide and the doping densities employed, doping fluctuations can play a significant role in the post-irradiation characteristics of the device.

Waveguide integrated graphene mid-infrared photodetector

Group IV platforms can operate at longer wavelengths due to their low material losses. By combining graphene and Si and Ge platforms, photodetection can be achieved by using graphene’s optical properties and coplanar integration methods. Here, we presented a waveguide coupled graphene photodetector operating at a wavelength of 3.8 μm.

Spatially controlled doping of silver in chalcogenide glass by thermal, photo and electron beam effects

For the first time, silver doping of bulk Gallium-Lanthanum-Sulphide (GLS) and Gallium-Lanthanum-Sulphur-Selenide (GLSSe) glass is demonstrated, presenting potential for a huge array of optoelectronic applications. Previously, silver doping of chalcogenide glasses were mostly based upon toxic, arsenic containing thin films [1-3]. GLS and GLSSe, however, offer a toxic free alternative, made by a safer and more economic technique, with infrared and nonlinear optic capacity [4].

Phase engineering of tin sulphide grown by atmospheric pressure chemical vapour deposition at ambient temperature

Considerable efforts have been made in the search for new photovoltaic (PV) materials that satisfy requirements such as low toxicity, optimum energy gap, high stability as well as the ability to synthesize by a wide range of methods. Tin (II) monosulfide (SnS), a binary metal (IV-VI) compound semiconductor offers many attributes. SnS is a p-type with high optical absorption coefficient >104 cm−1 and a band gap of Eg =1.1–1.32 eV making it a favourable material for PV applications and optical devices.

Structural modification of Ga-La-S glass for a new family of chalcogenides

In this work, the effect of adding Se, Te, In, Cs, Y to gallium lanthanum sulphide glass was studied. Structural modifications to the glassy network were achieved by substitution of sulphur, gallium or lanthanum using a melt-quench method in an inert atmosphere. Optical, thermal and mechanical characterisation of the samples revealed tailorable features according to the nature and the amount of glass modifier. In particular, the addition of selenium and tellurium resulted in an extended transmission in the infrared up to 12 μm. Furthermore, for small amounts of selenium, the position of the bandgap did not change significantly, maintaining visible transmission. The addition of indium led to the formation of glasses with longer transmission in the infrared and a cut-off edge around 600nm in the UV-visible range. Over-all, the addition of these modifiers resulted in stronger materials with improved thermal stability and similar mechanical properties to original Ga-La-S glass. The outcome of this work aims to bring a new family of chalcogenide glasses for applications in the infrared and visible range.