Study On Phase Change Material With Superlattice Structure

Phase change random access memory （PCRAM） is considered to be one of the mostpromising next generation information storage techniques. In the past several decades, thefootstep to develop advanced phase change materials has never been stopped, but so far,people can’t find any one of those materials to perfectly meet the needs ofcommercialization of PCRAM, the industrialization process is severely influenced by theconsiderably high operation current and the thermal crosstalk between adjacent phasechange cells. Phase change material with superlattice structure （SL） is put forward becauseof its unique structural and physical characteristics and has caused wide attention. Since theresearch in this field just begun, the theory of SL needs to be perfected. This thesis hassystematically studied on the preparation method, thermal conduction, electrical transport,surface state and other mechanisms of superlattices. In addition, the improvement of theperformance of phase change random access memory by using SL instead of conventionalphase change materials has been analyzed, and the feasibility of its application in logicdevice and multi-level storage has been discussed..By using the magnetic sputtering, GeTe and Sb₂Te₃films with high unformity havebeen deposited, the surface of the deposited film has atomic level flatness and theamorphous GeTe/Sb₂Te₃superlattice has been successfully prepared. Based on the opticalproperties of phase change materials, a self-setup testing system for phase changetemperature has been developed, the results indicates that the crystallization temperature ofSL is much higher than Ge₂Sb₂Te₅phase change materials, and is affected by the ratio ofthe component materials, period length and total thickness etc. The reason of thisphenomenon lies in the inhibition effect of the interfaces to the growth of the crystallinenucleus, also, the results of the x ray diffraction （XRD） experiment has approved the latticeaberration caused by the lattice mismatch.The thermal conductivity of SL is studied by a self-setup measurement system. Theresults show that the GeTe/Sb₂Te₃superlattices have a higher thermal conductivity, and withthe increase of the number of the interfaces, the thermal conductivity increases firstly and decreases afterwards. This result is in good agreement with the simulation by first principletheory and molecule dynamics, and0.13W/mK is the lowest thermal conduction of phasechange materials reported in papers. The results of the phonon spectroscopy calculationilluminates that the reduction of the phonon velocity, energy confine, interface thermalresistance and high frequency movement of the lattice vibration of the superlattice phasechange materials caused by the mismatch of the phonon modes can lead to the decrease ofits thermal conductivity. Based on the Raman scattering experiments, a “phonon modeannihilation” model is proposed to explain the low thermal conductivity. Finite elementsimulation indicates that the decrease of the thermal conductivity of phase change materialscan make the distribution of the current field and the temperature field more concentrated,so that the thermal crosstalk among adjacent memory cells can be diminished and thepower consumption of the PCM devices can be lowered. The testing of PCRAM cells hasproved this conclusion, and very short RESET time of0.8ns is achieved.The results of the four point probe method and the Hall Effect method show that theresistivity of the GeTe/Sb₂Te₃superlattice is higher than the Ge₂Sb₂Te₅phase changematerials, and is dominated by the carrier mobility. This property is propitious to reduce theRESET current. Furthermore, the temperature coefficient of resistance（TCR） of theGeTe/Sb₂Te₃superlattice increases from negative to positive with the increasing annealingtemperature, showing the difference with the “insulator-metal transformation”（IMT） of itscomponent materials and the highest IMT temperature （350℃³75℃）. A crystallite growthmodel based on the double schottky barrier（DSB） is proposed to explain its IMT effect,which is considered to be a result of the decrease of the grain boundary resistance（negativeTCR） and the increase of the grain resistance（positive TCR） in the annealing process, thisexplanation is approved by the impedance spectrum. The results of the optical reflectivitymeasurements, XRD and SEM have also illustrated that the grain size of the GeTe/Sb₂Te₃superlattice increases with the increase of the annealing temperature. Besides, because ofthe influence of the interface resistance, the temperature properties of resistance of thephase change material, such as IMT temperature can be ameliorated by adjusting thenumber of the interfaces.The results of the XPS and the positron annihilation spectroscopy of GeTe/Sb₂Te₃ superlattice show that the surface dangling bonds Te tends to lose electrons throughtunneling effect then become positive center, which inhibits its combination with O, thusshows different surface state from the Ge₂Sb₂Te₅phase change materials. Because of thesurface electrical field induced by the formation of Te dangling bonds, the discrepancy ofthe work function of GeTe/Sb₂Te₃superlattice in crystalline state and amorphous state ismuch bigger, which has been proved by electrostatic force microscope and XPS. Thus, thedifference of the conduction current between the two states of the heterojunction formed bySL can be larger, and this can be applied to improve the performance of a new phase changeheterojunction storage device. Because of the surface state, the charge carrier ofGeTe/Sb₂Te₃can change from p-type to n-type through cycle modulation. Therefore, a newphase change hererojunction can be formed without other semiconductor materials. Thismethod provides a new way to optimize the performance of the phase changeheterojunction devices. Moreover, because the phase change heterojunction based onGeTe/Sb₂Te₃superlattice can stably transform among three states: cut-off state with highresistance, conduction state with high resistance and conduction state with low resistance,so it can be applied in multi-level storage.