Study On The Phase Transition Kinetics Of GST Under Ultrafast Laser Excitation

Phase change material Ge₂Sb₂Te₅(GST)is one of the most important materials in high density optical phase change storage technology.In recent decades,the ultrafast transformation between crystal and amorphous has been widely studied.However,most of the current studies have focued on the initial and final states of the phase transition,but the dynamic process is not very detailed.When the ultrashort pulse laser is used as the induced laser,the dynamic process has a great influence on the ultrafast phase transition process.Therefore,the dynamic behavior of GST induced by the ultrashort pulse laser is studied in this paper.The innovation of this study is to establish a two-temperature equation considering the carrier density effect and the bipolar diffusion effect,and to expand the coupling between the space-time resolution and the first principle calculation,and to cooperate with the results of pump detection and ultrafast photocurrent test experiment.Finally a relatively complete GST phase transition dynamics process is proposed.Firstly,the carrier density evolution and non-equilibrium heat transport characteristics of GST materials during the phase transition induced by ultrashort pulse laser are studied.The research method is to build a double temperature model(TTM)for simulation calculation.Based on the absorption theory of semiconductor and the Boltzmann relaxation condition,a two temperature model is constructed,consisting of the density description equation of semiconductor carrier,the temperature description equation of carrier and lattice system is constructed.The innovation of this model lies in the consideration of carrier density and bipolar diffusion.The results show that:(1)The shortening of the pulse width of the incident laser will lead to the increase of the density of the photo generated carriers,the increase of the peak temperature of the carrier system,and the shortening of the time for the carrier and the lattice system to reach thermal equilibrium.(2)The density of photogenerated carriers has an obvious influence on the temperature evolution of the carrier system and the lattice system.Specifically,the first 40 picoseconds act as the medium of energy transmission between the carrier system and the lattice system,and then affect the temperature evolution of the lattice.But there is a minimum limit time of thermal equilibrium between the carrier and the lattice system about 80 picoseconds.(3)The bipolar diffusion effect can significantly reduce the density of free carriers and increase the crystallization and amorphization time of GST.The shorter the pulse width of the incident laser,the more significant the effect is.The defect of two temperature model and the first-principle calculation in the study of phase transformation dynamics of GST material induced by ultrashort pulse laser is studied.The research method is theoretical derivation and simulation calculation.The results are:(1)The temperature of the carrier system is the highest at the center of the incident laser irradiation,and the temperature decreases with the increase of the pulse width of the incident laser,but the decreased amplitude of the ultrashort laser pulse is much smaller than that of the long laser pulse.(2)The temperature of the carrier system decreases with the increase of the parallel distance from the irradiation center,but the decrease amplitude of the ultrashort pulse laser is faster than that of the long wave laser.(3)The power density response of incident laser with different pulse width is quite different.Then,the nonlinear optical absorption characteristics of GST material induced by ultrashort pulse laser are studied.The research method is to build an experimental platform for pump detection and an ultrafast photocurrent testing device for experimental detection.The results show that:(1)The initial absorption peak of GST reaches the maximum in 532 nm wavelength window and becomes stronger with the increase of incident laser power.(2)The density of state of GST is obtained by the first-principle calculation,and then the electron energy level corresponding to the 532 nm wavelength window is calculated in the range of 0.65 e V?3.82 e V.(3)In ultrafast photocurrent experiment,it is found that the time when the carrier density evolution curve reaches the initial absorption peak is about 600 fs,which is independent of the incident laser energy density and only related to the incident laser pulse width.(4)The time when the carrier density reaches the peak value is positively correlated with the pulse width of the incident laser.The extended research content of N-GST is the dynamic behavior change caused by doping modification.Finally,the reason why N-doping enhances the photon absorption effect of GST,but slows down the phase transition rate of GST is that although the nitride envelope enhances the photon absorption effect,part of the energy is used to break the Ge-N bond,and the low stress of the Ge-N bond can not give the Ge atom enough energy to make its transition,which disperses the energy originally used to break the Ge te bond,and hinders the ultrafast phase rate,although the nitride envelope enhances the photon absorption effect.Based on the above conclusions,it is proposed that the phase transformation dynamics of GST material induced by femtosecond pulse laser can be divided into four stages:(1)0?600 fs is the initial stage of laser energy deposition.The carrier system in GST is excited by light and coherent polarization occurs at tens of femtosecond.The characteristic spectrum of energy emission is lost due to electron collision,such as 532nm characteristic curve.The post photogenerated carrier density will peak at 500-600fs.At this stage,the lattice system is almost at room temperature.(2)600 fs?2 ps is a non-equilibrium plasma stage.The reason for this stage is that the electron lattice coupling time coefficient of GST is about 2 ps.In this stage,the carrier density begins to decrease,and the energy flow state of the carrier system changes from absorption to release,but most of the released energy is scattered in the form of radiation characteristic light waves and cannot be transferred to the lattice.In the period of 0?2ps,there will be a lot of Ge te bond breaking,Ge atom displacement and covalent bond weakening,and the shorter the incident laser wavelength,the stronger the effect of this stage.Because there is almost no thermal effect in this stage,and it can be considered as a complete non thermal process.(3)The period of 2?40 ps.In this stage,the carrier system transfers energy to the lattice through the radiating phonon,which can be considered as the main transmission medium.Although the lattice temperature has increased in this stage,it is only concentrated near the center point of laser energy irradiation,so it is impossible to observe the microstructure change at the macro scale.(4)After 40 ps,it is the period of energy coupling between the intrinsic carrier and the lattice.In this stage,the density of the photocarriers decreases to a very low level,so the energy transmission medium becomes the intrinsic carrier.From this stage,the phonon dynamics process of the lattice system begins to show gradually,and the material shows the characteristics of temperature and energy distribution,so this stage is also the main"thermal"phase transition process of GST.According to this conclusion,the deep reason that the fastest induced GST phase transition rate is short picosecond rather than femtosecond laser is explained,and it is pointed out that when the pulse width(attosecond)of ultrashort pulse laser is short enough to completely ignore the thermal process and dominated by the non thermal effect,the GST phase transition rate will be greatly improved.