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Abstract
The global use of portable electronic devices demands new non-volatile memories (NVM) with faster operation speed, better performance and higher capacity. One of the promising candidates for the next generation of NVMs is phase change random access memory (PCRAM) which offers lower power consumption, better scalability and faster data transfer rate. The principle of programing in PCRAMs is based on a thermally induced switching of phase change materials (PCMs) between an amorphous phase and crystalline state and the high electric resistance contrast between these phases. In this investigation, Ge2Sb2Te5 (GST) and Sb2Te have been selected as two potential candidates for PCRAM applications.
Firstly, the deposition of Sb2Te and GST films by pulsed laser deposition (PLD) was studied. The effects of substrate temperature and laser energy density on the structure, surface and stoichiometry of the films were investigated to enhance the quality of the films by (i) reducing the size and number of the large particulates, (ii) decreasing the roughness of the film and (iii) controlling the film stoichiometry. The results indicated that, by adjusting the laser energy density, the size and number of large particulates could be substantially reduced. Energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM) investigations show that substrate temperature significantly affects the stoichiometry of the films. The increase of the substrate temperature results in the reduction of the Te amount of the films. In addition to the stoichiometry, substrate temperature also has a significant impact on the roughness of the films (in the particulate free areas). The films deposited at the substrate temperature of 100 ℃ are partially crystallized; which leads to a dramatic increase in the surface roughness of the films.
Investigation on the isothermal growth velocity of Sb2Te crystals was performed to measure the activation energy for the growth velocity of Sb2Te crystals and study the growth mechanism of Sb2Te crystals in an amorphous matrix. The activation energy for the growth velocity was measured and compared with that of the GST. The growth velocity of Sb2Te crystals was found to be time-independent; suggesting an interface-controlled growth mechanism.
In and Pd were incorporated into Sb2Te film with the aim of improving the performance for PCRAM application such as archival life and programming speed. The effects of In and Pd doping on the crystallization behaviors, electrical properties and structural details of Sb2Te were investigated. The results indicated that the In-doped films with In content of 10.3 and 17 at%, show better data retention ability as compared to a conventional GST film. In addition, In doping raises the amorphous to crystalline phase resistance ratio which leads to the enhanced sensing margin. In incorporation also increases the crystallization rate which results in a faster programming speed. Pd doping does not significantly affect the data retention ability of Sb2Te, however, it increases the crystallization rate and reduces the resistance ratio of the amorphous phase to crystalline state. In addition, transmission electron microscopy (TEM) observations suggest that increase of dopant content in the films alters the crystallization mechanism from layerwise crystallization to random formation of crystalline nuclei.