The Optical And Electrical Properties Of Ge₂Sb₂Te₅ Thin Films In Nanoscale

In the emerging memory technologies, phase change random access memory (PCRAM) which possesses the advantages of fast switching, high density and low power consumption, is the most promising candidate for the next generation nonvolatile electronic memory. In order to be used as a core memory for portable electronic devices, the writing current of PCRAM should be minimized and its storage capacity should be increased. Two methods are usually used to enhance the memory capacity:one is to scale down the cell size, and the other is to store multistate in one cell. Impetus to data storage technologies has been given by scanning probe microscopy since nanoscale data recording is readily possible. In this thesis, we utilize conductive atomic force microscope (C-AFM) to analyze the electrical properties of Ge2Sb2Te5 film in nanoscale and demonstrate three-level data storage in amorphous GST thin film by C-AFM. The understanding of the crystallization process is of great importance for the preparation of Ge2Sb2Te5 films and the improvement of material properties. The morphological, optical and structural changes of GST films during the laser-induced crystallization process are investigated. The results of this thesis are as follows:1. The Ge2Sb2Te5 thin film was deposited by electron beam evaporation. Due to the high resolution and current sensitivity of C-AFM, the electrical properties of GST were investigated in nanoscale. When an IV-spectrum (0-10 V) is applied on the sample by C-AFM, GST thin film undergoes threshold switching at 2.0 V and memory switching at 7.2 V successively. The threshold switching is attributed to the conductive paths induced by high electric field, while the memory switching is due to the amorphous-to-crystalline phase change of GST material. With the bias voltage increasing, the GST film presents three well-resolved states (amorphous OFF state, amorphous ON state and crystalline state), each assigned as data values "0", "1" and "2" respectively. The electrical resistance of GST thin film decreases by over two orders of magnitude in both threshold and memory switching processes, providing clear contrast to distinguish between logical states. The threshold electrical field of threshold switching and the Joule heat accumulated for phase change were estimated.2. The Ⅳ characteristics of Ge2Sb2Tes film measured in C-AFM mode indicate that applying Ⅳ-spectra with lower voltage range (from 0-2 V to 0-7.2 V) can switch the amorphous material from high resistance state to low resistance state, while applying Ⅳ-spectra with wider voltage range (wider than 0-7.2 V) can switch the material from amorphous state to crystalline state. Nanoscale conductive marks in amorphous ON state and crystalline state were successfully fabricated by applying Ⅳ-spectra on the GST thin film.3. KrF pulsed excimer laser (46 mJ/cm2) was used to crystallize the as-deposited Ge2Sb2Te5 thin films. The AFM pictures of GST films show that the morphology of GST films changes with increasing number of the repetitions of the laser irradiation. Some spherical grains formed at the film surface after two times of laser irradiation, and then obvious sinking appeared on the film surface after three times of laser irradiation. The phenomena imply that the crystallization process switches from nucleation-dominated to crystal growth-dominated. The absorption coefficient and optical band gap of GST films were estimated through the transmittivity and reflectivity measured by spectrophotometer. The absorption coefficient of crystalline GST film is larger than that of as-deposited amorphous GST film, and the optical band gap of crystalline GST film is smaller than that of amorphous GST film. The Raman spectra of GST thin films before and after laser irradiation were investigated and for comparison. The variation of Raman spectra indicates a reduction of Ge-Te tetrahedral bonds and an increasement of Ge-Te octahedral bonds, while the Sb-Te bonds exhibit little change. The experimental data suggest that it is the transformation of Ge structure that results in the significant optical contrast between amorphous and crystalline GST thin films.