*4.3.1. Hafnium-based dielectrics*

HfO2 is a promising gate dielectric material due to its high dielectric constant and excellent thermal stability. Figure 28 shows three-dimensional DIGS plots for HfO2 atomic layer deposited on n-Si and over p-Si using chloride as metal (Hf) precursor. DIGS states are located at energies close to the majority band edge of the semiconductor. This can be explained in terms of the very nature of the conductance transient technique: majority band edges have the maximum majority carrier concentration, so states located at energies close to this position have the maximum probability to capture majority carriers. On the other hand, no conductance transients were observed for ultrathin samples (less than 40 Å). Kerber et al. [57] proposed the existence of a defect band in the HfO2 layer. We find spatially distributed defect bands for films on both types of silicon substrates. These defect bands could be due to oxygen vacancies: when the capacitor structure is terminated by the oxide-Si interface, the electric field existing in the dielectric film makes oxygen vacancies (positively charged) to move towards locations farther away from the interface. That occurs in samples deposited on n-type silicon f the difference in semiconductor band bending at the interface [58]. Forming gas annealings (FGA) are usually employed in integrated circuit technology for passivation of defects (dangling bonds) on Si surface. Figure 29(a) shows DIGS density corresponding to post-metallization annealed (400ºC, 30 min) Al/HfO2/p-Si sample. Lower DIGS density is achieved, but *Dit* density is increased in this sample [59], indicating that thermal treatment partially moves the insulator defects to the interface. Ioannou-Sougleridis et al. [60] attributed instabilities observed in as-grown Y2O3 samples to slow traps, which were mostly removed after FGA. The same behaviour can affect our results.

242 Dielectric Material

records).

We see that transients are decreasing when coming from accumulation and increasing when the sample is previously biased in inversion. At flat-band conditions traps previously charged (PCTs in accumulation and NCTs at inversion) can emit the trapped charge giving place to the corresponding flat-band voltage variation. We see also that these effects are more important as the setup time is higher indicating that trapping and detrapping are not instantaneous because the time needed by free carriers to reach the trap locations. Another important point is that decreasing and increasing transients seem to reach the same final values but after very long times (very much longer than those used in our experimental

The second case presented here is a sample in which the dielectric is a stack of an 21 nm HfO2 film grown by High-Pressure Reactive Sputtering (HPRS) and a SiO2 buffer layer (3.4 nm-thick). In this case (Figure 27), C-V curves indicate that at room temperature there is positive charge at the dielectric, that is PCTs predominates over NCTs. Consequently, in accumulation the positive charge increases and decreases in inversion regime, giving place to the counter clock-wise hysteresis cycle observed at room temperature. At 77 K the PCTs are not ionized and the hysteresis cycles are due only to NCTs and, then, a clock-wise hysteresis cycle is obtained. This model is confirmed by the opposite trends shown by the flat-band voltage transients obtained at room and low temperature (Figures 27(b) and (c)). Low temperature curves are similar to those obtained in the previous case (Figure 26).

In this section we review results obtained for several high-k dielectrics grown by atomic layer deposition (ALD) under different processing conditions. The most noticeable results

HfO2 is a promising gate dielectric material due to its high dielectric constant and excellent thermal stability. Figure 28 shows three-dimensional DIGS plots for HfO2 atomic layer deposited on n-Si and over p-Si using chloride as metal (Hf) precursor. DIGS states are located at energies close to the majority band edge of the semiconductor. This can be explained in terms of the very nature of the conductance transient technique: majority band edges have the maximum majority carrier concentration, so states located at energies close to this position have the maximum probability to capture majority carriers. On the other hand, no conductance transients were observed for ultrathin samples (less than 40 Å). Kerber et al. [57] proposed the existence of a defect band in the HfO2 layer. We find spatially distributed defect bands for films on both types of silicon substrates. These defect bands could be due to oxygen vacancies: when the capacitor structure is terminated by the oxide-Si interface, the electric field existing in the dielectric film makes oxygen vacancies (positively charged) to move towards locations farther away from the interface. That occurs in samples deposited on n-type silicon f the difference in semiconductor band bending at the interface [58]. Forming gas annealings (FGA) are usually employed in integrated circuit technology

**4.3. Conduction transient profiles of high-k dielectrics** 

provided by the experimental contour maps are outlined.

*4.3.1. Hafnium-based dielectrics* 

**Figure 27.** Normalized C-V curves and Flat-band voltage transients at room temperature (b) and 77 K (c) for an Al/HfO2/SiO2/n-Si sample grown by HPRS

Electrical Characterization of High-K Dielectric Gates for Microelectronic Devices 245

states densities are listed in Table 2. The measured value is similar in all samples, but non measurable at 500 ºC. It is possible that Al2O3 grown at this temperature is free of residual defects and moreover, the amorphousness, high purity and structural homogeneity achieved cause low defect densities, making the conductivity signal difficult to measure. In Figure 30 one can see the contour plot corresponding to the sample grown at 300 ºC. The shape is similar to HfO2 sample deposited on n-Si, but in the case of Al2O3 the maximum density appears near the interface which might cause faster defect detrapping. The highest quality sample in terms of DIGS states is that grown at 500 ºC, but if we consider also interface states densities obtained for these samples [62] the best sample would be that grown at 300 ºC. It is important to

consider both *Dit* and DIGS densities before concluding the quality of the samples.

300 12 400 19

600 15 800 25

**4.5. TiO2**

500 Undetectable

Growth temperature **Maximum DIGS ( 1010 cm-2 eV-1)**

**Table 2.** DIGS densities obtained to Al/Al2O3/n-Si structures grown at different temperatures.

**Figure 30.** Contour plot of DIGS density obtained to Al/Al2O3/n-Si (oxide grown at 300 ºC).)

TiO2 is being extensively studied for memory and logic applications, because of its high dielectric constant, ranging from 40 to 86. We have studied TiO2 atomic layer deposited on etched n-silicon and high-pressure reactive sputtered over SiO2–covered Si. DIGS state densities and other growth parameters are listed in Table 3. All ALD samples have been annealed at 750 ºC, so the only differences are growth temperature and chemical precursors. H2O seems to be more adequate as a precursor than H2O2 for the two grown temperatures. On the other hand, when titanium precursor is Ti(OC2H5), carbon remains uniformly distributed in the film bulk [63]. In contrast, when TiCl4 is used, chlorine remains in the film and accumulates near the interface [64]. Because of that, higher Dit and lower DIGS values

**Figure 28.** Three-dimensional DIGS plots for unannealed HfO2 atomic layer deposited on n-Si (a) and over p-Si (b)

Transition metal silicates, such as hafnium silicate, have also been the object of a considerable number of studies to replace SiO2 because of their higher crystallization temperature. Figure 29(b) shows DIGS states obtained from as-deposited Al/HfSixOy/n-Si structures grown using HfI4 and Si(OC2H5)4 as precursors. In this case contour lines have a more anisotropic shape than those for HfO2 indicating less homogeneous distribution of DIGS defects. In fact, we can see two different local ordering at zones A and B. The boundary between these zones approximately follows the line 588.22 15.42 *E E Csc T <sup>C</sup> x* . Contour lines are parallel in zone A and perpendicular to this boundary, indicating some regularity in the defect distribution. On the other hand, DIGS density rapidly decreases to lower values in zone B, where uniformity is higher. When this sample is submitted to a postdeposition annealing at temperatures ranging from 700 to 800ºC, this two-region structure does not change [61].

**Figure 29.** Contour plots of DIGS density obtained to 400 ºC-30 min. annealed Al/HfO2/p-Si (oxide grown at 450 ºC) and Al/HfSixOy/n-Si (silicate grown at 400 ºC)

#### **4.4. Al2O3**

The importance of Al2O3 as an insulating dielectrics is due to its large band gap (8.8 eV), excellent stability when deposited over silicon and its amorphousness (Al2O3 is a good glass former). We have studied Al/Al2O3/n-Si structures grown by atomic layer deposition at temperatures ranging from 300 ºC to 800 ºC. AlCl3 and H2O were used as precursors. DIGS states densities are listed in Table 2. The measured value is similar in all samples, but non measurable at 500 ºC. It is possible that Al2O3 grown at this temperature is free of residual defects and moreover, the amorphousness, high purity and structural homogeneity achieved cause low defect densities, making the conductivity signal difficult to measure. In Figure 30 one can see the contour plot corresponding to the sample grown at 300 ºC. The shape is similar to HfO2 sample deposited on n-Si, but in the case of Al2O3 the maximum density appears near the interface which might cause faster defect detrapping. The highest quality sample in terms of DIGS states is that grown at 500 ºC, but if we consider also interface states densities obtained for these samples [62] the best sample would be that grown at 300 ºC. It is important to consider both *Dit* and DIGS densities before concluding the quality of the samples.


**Table 2.** DIGS densities obtained to Al/Al2O3/n-Si structures grown at different temperatures.

**Figure 30.** Contour plot of DIGS density obtained to Al/Al2O3/n-Si (oxide grown at 300 ºC).)

#### **4.5. TiO2**

244 Dielectric Material

**ET-ECsc (meV)**

a)

over p-Si (b)

**30 32 34 36 38**

**Semiconductor conduction band**

**XC (Å)**

does not change [61].

**4.4. Al2O3**

**Figure 28.** Three-dimensional DIGS plots for unannealed HfO2 atomic layer deposited on n-Si (a) and

**-300 -250 -200 -150 -100 -50**

**ET-EVsc (meV)**

b)

**11,0 11,5 12,0 12,5 13,0 13,5 14,0 14,5**

**XC (Å)**

**<sup>0</sup> NDIGS (x1010cm-2**

**Semiconductor valence band )**

**eV-1**

**NDIGS (x1011cm-2**

**eV-1 )**

Transition metal silicates, such as hafnium silicate, have also been the object of a considerable number of studies to replace SiO2 because of their higher crystallization temperature. Figure 29(b) shows DIGS states obtained from as-deposited Al/HfSixOy/n-Si structures grown using HfI4 and Si(OC2H5)4 as precursors. In this case contour lines have a more anisotropic shape than those for HfO2 indicating less homogeneous distribution of DIGS defects. In fact, we can see two different local ordering at zones A and B. The boundary between these zones approximately follows the line 588.22 15.42 *E E Csc T <sup>C</sup> x* . Contour lines are parallel in zone A and perpendicular to this boundary, indicating some regularity in the defect distribution. On the other hand, DIGS density rapidly decreases to lower values in zone B, where uniformity is higher. When this sample is submitted to a postdeposition annealing at temperatures ranging from 700 to 800ºC, this two-region structure

**Figure 29.** Contour plots of DIGS density obtained to 400 ºC-30 min. annealed Al/HfO2/p-Si (oxide

The importance of Al2O3 as an insulating dielectrics is due to its large band gap (8.8 eV), excellent stability when deposited over silicon and its amorphousness (Al2O3 is a good glass former). We have studied Al/Al2O3/n-Si structures grown by atomic layer deposition at temperatures ranging from 300 ºC to 800 ºC. AlCl3 and H2O were used as precursors. DIGS

grown at 450 ºC) and Al/HfSixOy/n-Si (silicate grown at 400 ºC)

TiO2 is being extensively studied for memory and logic applications, because of its high dielectric constant, ranging from 40 to 86. We have studied TiO2 atomic layer deposited on etched n-silicon and high-pressure reactive sputtered over SiO2–covered Si. DIGS state densities and other growth parameters are listed in Table 3. All ALD samples have been annealed at 750 ºC, so the only differences are growth temperature and chemical precursors. H2O seems to be more adequate as a precursor than H2O2 for the two grown temperatures. On the other hand, when titanium precursor is Ti(OC2H5), carbon remains uniformly distributed in the film bulk [63]. In contrast, when TiCl4 is used, chlorine remains in the film and accumulates near the interface [64]. Because of that, higher Dit and lower DIGS values 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 inhibits trap displacements from the interface to the dielectric bulk.

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 this case.
