Research On The Preparation And Phase-change Properties Of Novel Sb-Te Based Films

Accurate research on phase-change material is critical for further PRAMtechnology development. Candidate materials for the application in PRAM mustfulfill several requirements: a good amorphous thermal stability, a short crystallizationtime, a low melting point, a high crystalline resistance and large resistance ratio, et al.Currently, in this paper, we try to look for solutions in terms of development andoptimization of novel phase-change materials in two parallel approaches: improvingGe2Sb2Te5material by doping other elements and developing novel Sb-Te basedphase-change materials. The main results are summarized as follows:1、 Zn-doped Ge2Sb2Te5phase-change materials have been investigatedsystematically. The proper Zn concentration (15.16at%) added into Ge2Sb2Te5servesas a center for suppression of the face-centered-cubic (FCC) phase to hexagonalclose-packed phase transition and FCC phase has high thermal stability partially dueto the bond recombination among Zn, Sb and Te atoms. Zn15.16(Ge2Sb2Te5)84.84filmexhibits a higher crystallization temperature (~258C), better data retention of10years at~167.5C.2、A binary Te-free ZnxSb100-xmaterial is proposed and developed. Films withlow Zn content (x<50at%) are crystallized in a single-step process with Sb while thefilm (Zn/Sb ratio is about1:1) exhibits two-step crystallization process fromamorphous to ZnSb metastable-phase and then to ZnSb stable-phase. Importantly, ithas higher crystallization temperature (~257C), larger crystalline activation energy(~5.63eV), better10-yr data-retention (~201C) and lower melting temperature(~500C).3、This study reports on the synthesis of novel tellurium-based chalcogenidefilms including Zn-Sb2Te3, Zn-Sb2Te, Zn-Sb3Te, Zn-Sb4Te and Zn-Sb7Te3. Theirstructural, thermal, optical and electrical properties are investigated. The results showthat the bond recombination among Zn, Sb and Te atoms can be observed in Zn-Sb-Tesystem. Sb-Te-based films with the addition of Zn can suppress the crystal grainsgrowth and exhibit a higher crystallization temperature, greater activation energy,better amorphous stability and data retention, faster crystallization speed, wider optical band gap, higher crystalline resistance and larger amorphous/crystallineresistance ratio. Compared with conventional Ge2Sb2Te5film, we can find the optimalcompositions of Zn-Sb-Te system are proposed to be about30at%Zn(Zn33.32(Sb2Te3)66.68, Zn38.82(Sb2Te3)61.18, Zn26.07(Sb2Te)73.93, Zn29.67(Sb2Te)70.33,Zn26.28(Sb3Te)73.72, Zn28.63(Sb4Te)71.37, Zn30.19(Sb7Te3)69.81). The crystallizationbehavior on a nano-second scale was observed using a static tester. The minimumtimes for crystallization of Zn30.19(Sb7Te3)69.81were revealed to be as short as~10ns ata given proper laser power of70mW.4、The Sb-rich Zn-Sb-Te films were prepared by doping high-stability ZnSb intoSb2Te3/Sb2Te films and properties measurements including structural, thermal, opticaland electrical properties have been investigated. Interestingly, the Zn content andSb/Te ratio have an important effect on the crystallization characteristics of Sb-richZn-Sb-Te films. In ZnSb-Sb2Te3system, the samples containing low Zn concentration(0.65,1.13and5.22at%) and Sb/Te ratio (0.78,0.86and0.95) exhibit a crystal grainof Sb2Te3and the samples containing high Zn concentration and Sb/Te ratio exhibit acrystal grain of Sb phase. In ZnSb-Sb2Te system, the samples containing low Znconcentration (3.27and6.66at%) and Sb/Te ratio (2.07and2.29) exhibit a crystalgrain of Sb2Te and the samples containing high Zn concentration and Sb/Te ratioexhibit a crystal grain of Sb phase. With the addition of ZnSb, a trade-off betweencrystallization speed and thermal stability can be obtained with the increase in Znconcentration and Sb/Te ratio. Especially, Zn19.73Sb48.06Te32.21and Zn28.62Sb53.69Te17.69exhibit both fast crystallization and good data retention. The reversible phasetransition can be performed by nano pulse laser irradiation tester.5、 Nano-composite ZnO-Sb2Te3materials have been systemically studied.ZnO-doped Sb2Te3films show better thermal stability than undoped Sb2Te3film. AsZnO content increases, the resistance in both amorphous and crystalline phasesimproves and there is four to five orders of magnitude difference in the resistancebetween amorphous and crystalline phase. In addition, the introduction of ZnO intoSb2Te3films can increase the optical band gap that is favorable to decrease thethreshold current of PRAM devices. XRD, Raman, XPS spectra show that a newSb2O3crystalline phase has formed due to the oxidation caused by O element in theair but this can be suppressed by the more ZnO addition (5.22at%) during thecrystallization process. The main reason is that the thermal stability is improved with more ZnO addition. The amorphous ZnO phase can be uniformly distributed aroundcrystal grain and suppress the crystal growth. According to the result of a static tester,the film with good thermal stability can perform the reversible phase transition andexhibit a faster crystallization speed than GST during the whole crystallizationprocess.