Nitrogen doping effect on tin-selenium thin films for phase change memory applications
Kamau, Charles Muthui
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Phase change materials have been widely used in optical and electronic storage due to rapid and reversible phase transition between amorphous and crystalline states with remarkable resistivity and optical contrasts. More recently these materials have been identified as suitable candidates for phase change random access memory (PCRAM) applications due to their potential for high scalability, low power and fast speed operation. Despite these superior properties and enormous potential, high reset currents necessitating high power consumption have been attributed to existing PCMs. Nitrogen doping has been proposed as a solution to these problems according to Caravati, et al., (2011). In this work, Nitrogen doped SnxSey phase change materials have been synthesized and characterized for optical, electrical and structural properties. The films were deposited by evaporation technique using Edwards Auto 306 Magnetron sputtering and evaporation system. Transmittance and reflectance data in the range 300-1700 nm were measured and calculated using Shimadzu 3700 DUV spectrophotometer for all the thin film samples. The optical measurements were simulated and analyzed using Scout software. Band gap values in the range 0.97 eV - 3.63 ± 0.005 eV were determined for SnSe thin films after doping with 0-5 sccm Nitrogen. Nitrogen doping was shown to increase the band gap in amorphous films with large increase seen between 0 sccm N2 and 1 sccm N2. X-ray diffraction (XRD) using XpertPro Panalytic XRD machine revealed the as-deposited films to be amorphous. The annealed films were found to be polycrystalline in nature with the number and height of peaks in the XRD scan increasing with increase in N2 doping. Sheet resistivity that was determined using Keithley source meter and a four-point probe set-up, was in the range 1150-1250±1 Ω-cm and 9.3- 0.3±0.05 Ω-cm for the amorphous and crystalline states respectively. Lowest sheet resistivity was observed at 3sccm N2 in amorphous SnSe films while crystalline resistivity gradually decreased as N2 doping was increased. The large change in resistivity of order 105 confirmed the phase change nature of the films. Nitrogen doping lowered the crystallization temperature from 107.8 ± 0.05oC at 0 sccm N2 to a minimum of 72.8±0.05 0C at 3sccm N2. From the I-V characteristics of the fabricated PCRAM cell, a threshold switching voltage, Vth of 4.47 V at a current 1.04 mA was obtained.