**4.6. Other materials: Mixtures**

Mixtures, ternary or quaternary oxides are also studied in order to find replacement for SiO2. Aluminum is a good glass former, so it can induce other dielectric layers to be amorphous, but at the expense of reducing the dielectric film permittivity. To avoid this fact, niobium is also mixed with dielectrics, due to its high permittivity. We have studied Hf-Al-O, Zr-Al-O, Hf-Al-Nb-O and Zr-Al-Nb-O mixtures.Ta2O5 layers have also been compared to Ta-Nb-O mixture. All these materials can be grown by ALD on p-silicon, using chlorides as precursors of hafnium and zirconium, Al(CH3)3 as aluminium precursor, and ethoxides for niobium and tantalum. Table 4 shows DIGS densities of these dielectric layers. In all cases niobium possibly acts as a barrier which inhibits trap displacement from the interface: in fact interface state densities are larger when Nb is incorporated and at the same time, DIGS state densities are reduced [65, 66]. Hf-Al-O behaves like Zr-Al-O due to the similarity between hafnium and zirconium. DIGS density for Ta2O5 has an intermediate value (~ 1011 cm-2 eV-1), as seen in the contour plot in Figure 32. By comparing this plot with Al/HfO2/p-Si plot, we realize that maximum DIGS reach deeper locations and lower energies for Ta2O5. This can be explained in terms of the larger valence band offset for HfO2 or ZrO2 with respect to Ta2O5.



Electrical Characterization of High-K Dielectric Gates for Microelectronic Devices 247

**Figure 31.** Contour plots of DIGS density obtained to ALD TiO2 sample grown at 225 ºC from TiCl4 on

etched silicon (a) and TiO2 sputtered on SiO2–covered silicon (600 ºC annealed) (b).

**Hf-Al-O** 1200 **Hf-Nb-Al-O** 2 **Zr-Al-O** 2000

**Ta2O5** 120

**5. Conclusions and future trends** 

**Zr-Nb-Al-O** Not detected

**Ta-Nb-O** Not detected **Table 4.** DIGS densities obtained for different high-k dielectric mixtures

**Maximum DIGS (x109 cm-2 eV-1)**

**Figure 32.** Contour plot of DIGS density obtained to Al/Ta2O5/p-Si (oxide grown at 300 ºC).

In this chapter we review several experimental techniques which allow detecting, measuring and identifying traps and defects in metal insulator interface, and at the bulk of the dielectric. The correlation between conduction mechanisms, defect location and preferential energy values provides very relevant information about the very nature of defects and, eventually, how these defects could be removed or diminished. Our techniques

**Table 3.** DIGS densities obtained to TiO2 deposited over n-silicon and over SiO2

**Figure 31.** Contour plots of DIGS density obtained to ALD TiO2 sample grown at 225 ºC from TiCl4 on etched silicon (a) and TiO2 sputtered on SiO2–covered silicon (600 ºC annealed) (b).


**Table 4.** DIGS densities obtained for different high-k dielectric mixtures

**Figure 32.** Contour plot of DIGS density obtained to Al/Ta2O5/p-Si (oxide grown at 300 ºC).

#### **5. Conclusions and future trends**

246 Dielectric Material

this case.

**4.6. Other materials: Mixtures** 

are seen in the films grown with TiCl4. To compare with the previous results, we grew TiO2/SiO2 dielectric thin films stacks on n-type silicon substrates. A 7 nm layer of SiO2 was deposited by an Electron Cyclotron Resonance (ECR) oxygen plasma oxidation. Afterwards, 77.5 nm TiO2 films were grown in an HPRS system at a pressure of 1 mbar during 3 hours and at a temperature of 200ºC. Finally, some samples were *in situ* annealed in oxygen atmosphere at temperatures ranging from 600 to 900ºC. Sputtered films exhibit lower DIGS densities, but the large band gap buffer layer (SiO2) interposed between substrate and TiO2

Figure 31 shows two contour maps corresponding to ALD sample grown from TiO2 (Figure 31(a)) and to sputtered (600 ºC annealed) sample (Figure 31(b)). Defects are located closer to the interface in ALD films because the wider band gap SiO2 interface layer is not present in

Mixtures, ternary or quaternary oxides are also studied in order to find replacement for SiO2. Aluminum is a good glass former, so it can induce other dielectric layers to be amorphous, but at the expense of reducing the dielectric film permittivity. To avoid this fact, niobium is also mixed with dielectrics, due to its high permittivity. We have studied Hf-Al-O, Zr-Al-O, Hf-Al-Nb-O and Zr-Al-Nb-O mixtures.Ta2O5 layers have also been compared to Ta-Nb-O mixture. All these materials can be grown by ALD on p-silicon, using chlorides as precursors of hafnium and zirconium, Al(CH3)3 as aluminium precursor, and ethoxides for niobium and tantalum. Table 4 shows DIGS densities of these dielectric layers. In all cases niobium possibly acts as a barrier which inhibits trap displacement from the interface: in fact interface state densities are larger when Nb is incorporated and at the same time, DIGS state densities are reduced [65, 66]. Hf-Al-O behaves like Zr-Al-O due to the similarity between hafnium and zirconium. DIGS density for Ta2O5 has an intermediate value (~ 1011 cm-2 eV-1), as seen in the contour plot in Figure 32. By comparing this plot with Al/HfO2/p-Si plot, we realize that maximum DIGS reach deeper locations and lower energies for Ta2O5. This can be explained in terms of the larger valence band offset for HfO2 or ZrO2 with respect to Ta2O5.

**TiO2 atomic layer deposited over n-Si TiO2 sputtered over SiO2**

Annealing Maximum DIGS

(1011 cm-2 eV-1)

(1011 cm-2 eV-1)

Ti(OC2H5), H2O2 225 3,5 600 ºC 0,5 Ti(OC2H5), H2O2 275 1 700 ºC 2,6 TiCl4, H2O 225 2 800 ºC 1,2

**Table 3.** DIGS densities obtained to TiO2 deposited over n-silicon and over SiO2

Ti(OC2H5), H2O 275 0,1 No Not detected

900ºC Not detected

Precursors TG (ºC) Maximum DIGS

inhibits trap displacements from the interface to the dielectric bulk.

In this chapter we review several experimental techniques which allow detecting, measuring and identifying traps and defects in metal insulator interface, and at the bulk of the dielectric. The correlation between conduction mechanisms, defect location and preferential energy values provides very relevant information about the very nature of defects and, eventually, how these defects could be removed or diminished. Our techniques

#### 248 Dielectric Material

provide high resolution in two dimensions: defect energy (E) and depth relative to the interface (z). In the future, we want to combine these techniques with scanning probe microscopy in order to obtain high resolution in lateral dimensions (x,y) as well.

Electrical Characterization of High-K Dielectric Gates for Microelectronic Devices 249

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