Effect of Zn Doping On In2se3 Thin Films for Phase Change Random Access Memory (Pram) Applications
Opiyo, Samwel Olaka
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Phase Change Memory is one of the most promising concepts for the future generations of Non Volatile Memories. Extensive work is nowadays performed on the Phase Change Materials to optimize the cell properties for specific applications. In particular, for the embedded memory market, it is desirable to reduce the writing current while raising data retention capability. In2Se3 has shown remarkable characteristics for phase change random access memory (PRAM) applications. Among the remarkable characteristics is high resistance contrast of about 5 orders of magnitude for amorphous film as compared to crystalline film. In this study the effect of Zinc doping on electrical characteristics of In2Se3 for PRAM applications was investigated. The samples were synthesized in the specified ratios (In:Se = 2:3) by mass. Zn was introduced as a percentage (0%, 2%, 4%, 6%, and 8%) of the final composition attained. Edward Auto 306 RF/DC vacuum evaporation chamber and Keithley 2400 sourcemeter were employed for thin film deposition and sheet resistance determination respectively. Thermal cycling of the thin films was done using Lindbergh furnace. The results obtained show considerable effect of zinc doping on sheet resistance for as deposited (2.13x108, 2.199x108, 2.599x108, 2.648x108, 2.756x108 and 3.227x108 Ω/sqr) and annealed films (1.590x103, 1.629 x103, 1.883 x103, 1.988 x103, 2.493 x103 and 3.443 x 103 Ω/sqr). In-situ electrical properties during thermal cycling indicated a sharp drop in sheet resistance at the crystallization temperature showing a transition from amorphous to crystalline phase. Zinc doping also affected the resistance contrast with the highest and the lowest contrasts found to be 1.38x105 Ω/Square and 9.37x104 Ω/Square for 4% and 8% Zn doping respectively. Crystallization temperature also increased linearly with zinc doping (107.28, 114.63, 121.72, 141.60 and 147.51 for 0%, 2%, 4%, 6% and 8% Zn doping respectively).4% Zn doped sample was the best as it registered the highest resistance contrast and moderately high crystallization temperature. The activation energy of 4% Zn doped sample was found to be 0.890 + 0.048 eV. The fabricated PRAM registered a threshold voltage of 4.60V during I-V testing. The pulsed mode testing resulted in SET pulse of 2.38V, 15μs and a RESET pulse of 4.75V, 75ns. The increased threshold voltage as well as activation energy suggest that this material is scalable.