Investigation of Phase Change Material Threshold Voltage for RF Switches and Other Applications
Open AccessPhase change materials (PCMs) have been used in multiple applications including in rewritable CD/DVDs, random access phase change memories, and more recently as a switching element in RF systems. The application of phase change materials as an RF switch brings with many challenges including the transition mechanism, reliability, achieving OFF/ON ratio, and others. However, chief among them is their ability to block RF signals. A feature, threshold voltage switching, very much desired in phase change memories, could be detrimental in RF applications. This is because when RF signals are applied to a switch in the off state, an equivalent DC voltage equal to the rms value of the RF signal develops across the switch, and if this rms voltage exceeds the threshold voltage it can transition the PCM to the conductive state prematurely.Given that the threshold voltage is a phenomenon of critical importance to the feasibility of using PCM as an RF switch, this work focuses on the investigation of phase change threshold voltage. In particular, the influence of an electric field in the amorphous to crystalline transition is investigated. In this work models for growth and nucleation rates of Germanium Telluride (GeTe) are developed and validated with measured data as well as with data previously published. Once the models are validated, an electric field component is incorporated using previously published theoretical treatments. The models are then used to access the effect of an applied electric field to both the growth and nucleation rates.Based on the models, it appears that if the electric fields influence the nucleation rate but have little to no effect on the growth rate. Given that it appears that the electric field does seems to have an influence, and structure was fabricated to study its effects on the amorphous to crystalline transition. The structure was designed to allow for the simultaneous application of both an electric field directly to the PCM, and a heat pulse from and external heater. This design allows for the transition of the PCM using only an external heat pulse or a combination of a heat pulse and an electric field. To avoid the rapid amorphous to crystalline transition that happens when the threshold fields is exceeded in the PCM, this work focused the investigation on the effect that subthreshold electric fields would have on the amorphous to crystalline transition. To study the effects of subthreshold electric fields, the threshold voltage was first measured to make sure the voltages applied were below threshold. Once the threshold voltage was known, the next step was to find the minimum energy needed to transition the PCM only using the external heat pulse. The PCM is then transition with various subthreshold voltages applied and the energy from the heater in addition to the energy from the applied voltage due to leakage is compared to the energy of the heater without any fields applied. The data indicated that the heater energy can be reduce by more than 50% with negligible energy due to leakage current. It was also found that there exist a critical subthreshold electric field of about 5.6V/um that to the best of our knowledge has not been previously reported, and that appear to be responsible for the onset of the electric field effect on the amorphous to crystalline transition.
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DeLaCruz_gwu_0075A_16608.pdf | 2024-01-11 | Open Access |
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