Formation of CoSi 2 nanocrystal after annealing

From TEM image was showed in the Fig. 4-7 and that show the cross-sectional TEM and the C-V hysteresis of the fabricated device sample after RTO process.

Co-sputtered film became two separate ports in trapping layer and the C-V hysteresis which there is large memory window indicates that trapping effect occurs. At high temperature condition and in the controlled-atmosphere chamber filled with oxygen at atmospheric pressure, silicon, germanium and cobalt will easily react with oxygen to form oxide. We also know that aggregation generally occurs at higher temperatures [4-8]. Fig. 4-10 also shows the cross-sectional TEM and the C-V hysteresis of the fabricated device sample after RTO process. The difference in temperature between Fig. 4-7 and Fig. 4-10 there is two hundred degrees. The TEM image in Fig. 4-7

shows the sample which was annealed in lower temperature. Besides, co-sputtered film became two separate ports in trapping layer. One of them is clearly observed that many NCs formed under continuous film and on tunnel oxide by thermal treatment.

We could observe that the trapping effect occurred from its C-V hysteresis. Compare this with Fig. 4-10, we could find that there are no separate layers and NCs in this trapping layer after annealing in higher temperature. Besides, there was smaller but not zero memory window in its C-V hysteresis. Because of them, we could consider the main trapping effect occurs on the nanocrystal and the oxidized Ge elements contribute extra charge trap site.

We could know that cobalt oxide, germanium oxide and silicon oxide will be formed by chemical reactions in the Co- Si0.5Ge0.5 co-sputtered thin film and CoSi2

will become agglomerate during the annealing process. The TEM image in Fig. 4-7 shows the sample which was annealed in lower temperature. We could observe that there are two layers in the Co- Si0.5Ge0.5co-sputtered thin film. The upper layer is a continuous film which is nonconductors and the lower layer is a discontinuous layer which is formed with many NCs and we can’t observe it in Fig. 4-10 which was annealed after annealing in higher temperature.

We could understand the mechanism of CoSi2 aggregation and oxidation. First, Co elements easily diffuse to silicon dioxide such as tunnel oxide or capped oxide.

During the thermal process, the diffusion behavior of Ge and Co dominates the aggregation of nanocrystals. We know that that various elements will de agglomerate and oxidize simultaneously. In upper layer, Co, Si and Ge has been oxidized to form various oxide before agglomerating and form a capped oxide which is similar to silicon dioxide with various oxide such as CoOx , SiOy and GeOz. It can be found that Ge and Co tend to diffuse out to the capped oxide obviously. The out-diffused Co and Ge react with external oxygen to form the cobalt oxide and germanium oxide during

the process of RTO rapidly. Because there is a capped oxide which is similar to silicon dioxide in the bottom layer, the driving force for the diffusion of oxygen interstitials into the Co-SiGe co-sputtered film would be the solubility limit of oxygen at the anneal temperature. On the other hand, the aggregative growth rate will be greater than oxidation rate in the bottom trapping layer. From the analysis of material in Fig.

4-9, we could understand that there are molecules with Ge-O, Co-O and Co-Si bonds in this trapping layer. The same thing may be said of Fig. 4-7. The XPS analysis in Fig. 4-9 is agreement with observation in Fig. 4-7.

From TEM image was showed in the Fig. 4-10 and that show the cross-sectional TEM and the C-V hysteresis of the fabricated device sample after RTO process. From Fig. 4-12 showed the XPS analysis of the trapping layer after annealing at higher temperature, we could know that there are molecules with Ge-O and Co-O but no Co-Si bonds in this trapping layer. The same thing may be said of this TEM image and the XPS analysis of this trapping layer. The oxidation rate will be greater than aggregative growth rate in the bottom trapping layer after annealing higher temperature.

Fig. 4-13 showed the electrical current density-voltage (J-V) hysteresis of three conditions differed from oxidation duration of the Co- Si0.5Ge0.5 co-sputtered thin film.

In Fig. 4-13, we could observed that the leakage currents exhibit a nearly result about from 10^-12 order to 10^-13 order. As the sample was oxidized without capped oxide layer, shown in Fig. 4-15, the Ge elements without any block were out-diffused seriously. The cobalt elements react with Silicon to form CoSi2 and with oxygen to form CoO. CoSi2 will agglomerate when the aggregative growth rate was greater than oxidation rate in the bottom trapping layer.

Si-substrate tunnel oxide

Si Ge

Co

O₂ O₂ O₂ SiOx+GeÆGeO+SiO_y

Si-substrate CoO

GeO₂ SiO₂

tunnel oxide CoSi₂、Si、SiO_x Co

Ge SiOx+GeÆGeO+SiO_y

CoO

GeO₂ SiO₂

Si-substrate CoSi₂、Si、SiO_x Co

Ge tunnel oxide

Figure 4-15 The oxidation and aggregative mechanism in the Co-

Si

Ge

co-sputtered thin film without capped oxide

4.3 The role of capped oxide during the formation of