Abstract
Materials design for phase-change memory (PCM) has been testified on heater/diffusion-barrier layer (TaSi2Nx, TiSi2Nx), new recording media (GaSbTe & GeSn) and wet etching solution for Ge2Sb2Te5.Highly resistive Si2TaNx and Si2TiNx amorphous films were studied aiming at their use as heating/diffusion-barrier layer for PCM. The measured electrical resistivity of Si2TaNx and Si2TiNx films between 0.069 ~ 1.21 and 0.039 ~0.69 ohm-cm, respectively, fulfills the requirements of suitable heating layer suggested by simulations and is determined by nitrogen content which is tunable up to 52.8 % and 38.68 %, respectively. The Si2TaNx and Si2TiNx films with higher N contents showed excellent thermal stability with amorphous structure sustaining until at least 800 oC. However, these samples exhibited a high degree of temperature dependent resistivity (TCR), and showed a negative TCR at 400~500 oC. The binding energy of Si2TaNx and Si2TiNx films changes with N content and Si/(Si+Ta) (or Si/(Si+Ti)) ratio, conforming with the performance of electrical resistivity. Meanwhile, the highly resistive Si2TaNx and Si2TiNx heating layers successfully obstructed the diffusion of tungsten atoms from the W electrodes even with quite a thin layer of only 10 nm. Optimal composition of Si2TaNx or Si2TiNx film as a heating and diffusion-barrier layer for PCM was proposed.Novel materials based on Ga-Sb-Te system are proposed for PCM. Studied compositions were located along the pseudo-binary tie-line GaSb-Sb8Te2, in which atomic ratio Sb/Te of GaSbTe films is higher by incorporating GaSb into Sb8Te2 indicating a possible higher crystallization speed hereof. The GaSbTe films were explored by systematic evaluations of crystallinity, crystallization and melting temperatures (Tx, Tm), resistivity at crystalline and amorphous states (Rc, Ra), Ra/Rc ratio and temperature dependent electrical resistivity for PCM. Crystallization kinetics of these films was studied by measuring the temperature dependent electrical resistance during non-isothermal heating. The activation energy (Ea), rate factor (Ko) and kinetics exponent (n) were deduced from Kissinger’s plot and Ozawa plot, respectively. The crystallization time of each composition was evaluated using JMA equation within all given kinetics parameters. The Composition M (Ga3Sb8Te1) exhibiting single endothermic peak, lower Tm, higher Tx/Tm and Ra/Rc ratios, suitable Tx and electrical resistivity at crystalline states and the shortest crystallization time among studied ones is highly potential for PCM to reduce the reset current.We have proposed amorphous Ge1-xSnx films with new functionality for use in PCM due to appropriate compositional tunability for extending the solid solubility. The memory effect is accomplished by, after appropriate heating, an amorphous to crystalline transition which was found to be reversible. Sn content determines the order of decrease in electrical resistivity before and after phase-transition. Ge60Sn40 and Ge72Sn28 films exhibited a four-order and two-order of magnitude decrease in resistivity, respectively. The mechanism of resistivity dropt in Ge72Sn28 film with a two-step phase-transition was investigated and the crystallization kinetics was also demonstrated. GeSn films are found to be potential candidates for PCM.The Ge2Sb2Te5 thin film can be control-etched by an aqueous solution of 20% nitric acids (HNO3). It was found that the Ge2Sb2Te5 films in amorphous state could be easier etched more uniformly than that in crystalline state. The wet-etching of amorphous Ge2Sb2Te5 films was studied by ICP and XPS spectrometry. It is deduced to arise from chemical etching that starts with bond breakages, oxidation of each element and subsequent dissolution of the resultant oxides. The Ge element is easier debonded from the Ge-Sb-Te matrix than is Te but Ge oxide is more stable than Te oxide. Te element is relatively sluggish debonded than Ge, yet Te oxide is quite unstable. As a result Ge is the first leached element that dominates etching process. Sb is the most difficultly leached element in Ge2Sb2Te5 thin films. Test cells of PCM were successfully manufactured using the wet-etching process, and studies of the switching properties revealed a low threshold voltage of 0.60 ± 0.15 V.