Abstract
We present the enhanced properties observed in the phase change memory alloy Ge2Sb2Te5 (GST) when doped with arsenic. Although arsenic is known as a toxic element, our observations show that significant improvement can be obtained in GST systems on thermal stability, transition temperature between amorphous and crystalline phases and switching behaviors when doping with arsenic. Though both the GST and arsenic doped GST are amorphous in the as-deposited state, only GST alloy turns to crystalline NaCl-type structure after annealing at 150 °C for 1 h. Results from the resistance versus temperature study show a systematic increase in the transition temperature and resistivity in the amorphous and crystalline states when the arsenic percentage in the GST alloy increases. The crystallization temperature (Tc) of (GST)0.85As0.15 is higher than the Tc observed in GST. Optical band gap (Eopt) values of the as-deposited films show a clear increasing trend; 0.6 eV for GST to 0.76 eV for (GST)0.85As0.15. The decreases in Eopt for the samples annealed at higher temperatures shows significant optical contrast between the as-deposited and annealed samples. Though all (GST)1−xAsx alloys show memory switching behaviors, threshold switching voltages (VT) of the studied alloys show an increasing trend with arsenic doping. For (GST)0.85As0.15, VT is about 5.2 V, which is higher than GST (4.0 V). Higher transition temperature and higher threshold switching values show arsenic doping in GST can enhance the memory device properties by improving the thermal stability and data readability. Understanding the doping effect on the GST is important to understand its crystallization properties. Structure properties of amorphous GST, Ge2Sb2−0.3As0.3Te5 and (GST)0.85As0.15 models were studied using first principles molecular dynamics simulations, compared their partial radial distribution functions, and q parameter order. Arsenic doping into GST features interesting structural and electronic effects revealed by the radial distribution functions, q order parameter and band gap value, in line with the experimental findings.
Highlights
Arsenic is known as a toxic element, our observations show that significant improvement can be obtained in Ge2Sb2−0.3As0.3Te5 and (GST) systems on thermal stability, transition temperature between amorphous and crystalline phases and switching behaviors when doping with arsenic
We focused on three model systems containing 459 atoms: (i) The first model corresponds to the reference Ge2Sb2Te5 (102 Ge, 102 Sb and 255 Te atoms) material, referred to as GST. (ii) The second model is for a doped alloy obtained by direct substitution of Sb by As atoms (102 Ge, 15 As, 87 Sb and 255 Te atoms) that corresponds to 3% of As and is referred to by the formula Ge2Sb2−0.3As0.3Te5. (iii) The third model is for a doped alloy obtained by proportional replacement of the Ge, Sb and Te atoms by As according to its pure GST relative composition resulting in (GST)1−xAsx with x = 15% (86 Ge, 72 As, 86 Sb, 215 Te)
The dependence on As content on the optical and electrical properties of (GST)1−xAsx films were studied through the structural transition by annealing at different crystallization temperature
Summary
We present the enhanced properties observed in the phase change memory alloy Ge2Sb2Te5 (GST) when doped with arsenic. Arsenic is known as a toxic element, our observations show that significant improvement can be obtained in GST systems on thermal stability, transition temperature between amorphous and crystalline phases and switching behaviors when doping with arsenic. Higher transition temperature and higher threshold switching values show arsenic doping in GST can enhance the memory device properties by improving the thermal stability and data readability. Ge-Sb-Te (GST) alloys are phase-change materials that possess superior properties for memory applications and are currently used in optical and electrical rewritable data storage devices. Many ongoing studies aim to improve the material properties by doping with suitable elements and to overcome practical application oriented problems like high reset current, crystallization speed, thermal stability in the amorphous state, etc[8,9,10,11].
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