Recently, rare earth oxides such as La-based binary or ternary compounds, have received a lot of attention for their potential use as second generation high dielectric constant materials instead of the currently used Hf-based dielectrics in complementary metal–oxide–semiconductor technology.^[1–5] Among these rare earth oxides, lanthanum oxide is considered to be a potential candidate due to its superior properties such as a high dielectric constant (> 26), large band gap (> 5.8 eV) and high electrical breakdown field strength.^[6,7] It is also considered to overcome the low crystallization temperature problems with other high- dielectrics such as HfO₂, as it has a high crystallization temperature of > 900 C.^[8,9] To meet the requirements for the dielectric quality improving and thickness scaling, atomic layer deposition (ALD) has become the preferred industrial deposition technique for high- oxides because of its low deposition temperature, excellent thickness and composition control, and conformal growth.^[10] However, due to the well-known hygroscopic nature of La , H₂O used as an oxidant in ALD process can negatively affect the La growth step, and consequently also the properties of the thin film. To solve the above problems, ultraviolet ozone post treatment, capping layer, and other metal elements such as Al and Zr adopted into La have been studied to suppress the moisture absorption.^[11–13] Besides these methods, other metal elements incorporated into La shows advantages in the scaling of thickness and in the controlling of properties such as dielectric constant, corrosion resistance and thermal stability. However, the oxidants’ (H₂O or O effects on the characteristics of metal elements incorporated with La have not been discussed. Different oxidants used in the ALD process have a great influence on the defects and residual impurities in the high- films if the process conditions are optimized.^[10,14] Since this topic can be crucial for the successful inclusion of high- dielectrics in electron devices, the oxidant effects on the characteristics of metal element adopted La-based ternary compounds should be investigated.

In this paper, different oxidants (H₂O and O are used to deposit La /Al nanolaminates by the periodic stacking of bilayer structures on Si substrate by using the ALD technique at 300 °C. The effects of different oxidants on the physical properties and electrical characteristics of La /Al nanolaminates are investigated after annealing.

Prior to the deposition, 8–12 cm P-type Si (100) wafers were RCA cleaned and a 30-s dip in diluted HF solution was used to remove the native oxide, followed by a 60-s rinse in deionized water. Tris(isopropylcyclopentadienyl) lanthanum[La( PrCp)₃] was used as the lanthanum precursor and trimethylaluminum (TMA) was utilized as the aluminum precursor. The container of lanthanum precursor was heated to 180 °C and aluminum precursor was at room temperature, corresponding to a vapor pressure of 10 hPa to 15 hPa. When H₂O was used as the oxidant, the container was set at room temperature, corresponding to a vapor pressure of 7 hPa. The ozone generator used the ultra pure O₂ (99.999%) to obtain O₃, the concentration was 200 g/Nm . La /Al nanolaminates were formed by the periodic stacking of bilayer structures to form thicker multilayers. Different thickness La /Al nanolaminates were deposited on Si substrates by ALD using different metal processors (La( PrCp)₃, TMA) and oxidants (O₃, H₂O) at 300 °C. After all the nanolaminates were annealed in the nitrogen at 600 °C for 60 s, the x-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM) and electrical measurements were carried out to investigate the characteristics of the different processed La /Al nanolaminates.

Figure 1 shows the XPS spectra of the La /Al nanolaminates grown using different oxidants before and after annealing. As shown in Fig. 1(a), for H₂O-based ALD process, the main peaks are La, Al, and O, the subordinate peak is C. The remaining peaks are mainly attributed to the Si substrate and Auger peaks. An obvious change is observed in the spectrum after annealing. For the O₃-based ALD process as shown in Fig. 1(b), the main peaks are La, Al, and O, and the subordinate peak is C. The remaining peaks are mainly attributed to the Si substrate and Auger peaks. After annealing, the variation in the spectrum is negligible. The results indicate that La /Al nanolaminates with low impurity levels are obtained by ALD using different oxidants. A small composition variation of O₃-based La /Al nanolaminate is observed compared with that of H₂O-based La /Al nanolaminate after annealing.

Fig. 1.

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	Fig. 1. (color online) XPS spectra of La /Al nanolaminates grown using different oxidants before and after annealing.

The composition variations of XPS spectra for the La /Al nanolaminates before and after annealing can be explained by the percentage composition of atomic atom variation, which is shown in Table 1. For the H₂O-based ALD process, the percentage composition of atomic atom ratio of La:Al:O is close to 1:1:3 and it changes to almost 1:2:5 after annealing. Meanwhile, the C impurity decreases from 2.3 to 1.24 after annealing. The excess oxygen after annealing may be attributed to the hygroscopicity of the La-based material leading to the incorporation of the related OH groups. For the O₃-based ALD process, the percentage compositions of atomic atoms ratio of La:Al:O are both close to 1:1:3 before and after annealing. The C impurity decreases from 1.3 to 0.69 after annealing. The results indicate that stoichiometric La /Al nanolaminates are deposited for both O₃ and H₂O as oxidants if the deposition process is optimized. After annealing, the difference may be explained as follows. Owing to H₂O tending to physisorb on the surface strongly and contributing to a high concentration hydroxyl/hydrogen groups in the films although the purge time is sufficiently long,^[14] the intermixing reaction occurs during the annealing, accompanied by the decomposition and re-composition of unstable bonds or groups residing in the nanolaminates. However, O₃ is easy to purge and no hydroxyl species reside in the films compared with H₂O. Therefore, the obvious change is observed in the XPS spectra.

Table 1.

Table 1.

Table 1. Percentage compositions of atomic atoms in La2O3/Al2O3 nanolaminates. . La /at.% Al /at.% O /at.% C /at.% N /at.% H2O-based As-dep. 17.94 20.27 59.49 2.30 0 anneal 12.23 22.95 63.58 1.24 0 O3-based As-dep. 19.23 21.86 57.61 1.30 0 anneal 21.50 21.00 56.81 0.69 0

Table 1. Percentage compositions of atomic atoms in La2O3/Al2O3 nanolaminates. .

Figure 2 shows the O spectra of La /Al nanolaminates deposited by ALD using different oxidants. For H₂O-based ALD process, the O core level spectrum consists of five fitting peaks, which are located at binding energies of 529.18 eV, 530.18 eV, 531.13 eV, 532.28 eV, and 533.17 eV. These peaks correspond to the chemical bonds of La–O–La, La–O–Al, Al–O–Al, La–O–H, and O–C, respectively.^[11] After annealing, the peak intensity of La–O–H chemical bond increases obviously and the peak of O–C chemical bond diminishes. It can be concluded that the formation of La(OH) occurs during annealing due to the residue of hydroxyl/hydrogen groups in the layers. For O₃-based ALD process, the O core level spectrum consists of three fitting peaks, which are located at the binding energies of 528.94 eV, 530.40 eV, and 531.68 eV, respectively. These peaks are corresponding to the chemical bonds of La–O–La, La–O–Al, and Al–O–Al, respectively.^[11] After annealing, no obvious change is observed except the variation of peak intensity due to the diffusion of atoms.

Fig. 2.

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	Fig. 2. (color online) O spectra of La /Al nanolaminates deposited by ALD using different oxidants.

Figure 3 shows the surface roughnesses of the La /Al nanolaminates before and after annealing. As shown in Fig. 3, the root mean square (RMS) roughness of the H₂O-based nanolaminate increases sharply from 0.337 nm to 1.831 nm after annealing, whereas that of the O₃-based nanolaminate decreases from 0.332 nm to 0.072 nm. The RMS value of H₂O-based nanolaminate increases nearly 5.4 times after annealing possibly due to the formation of low density hexagonal La(OH)₃.^[15] Meanwhile, due to the densification of the nanolaminate after annealing, the RMS value of O₃-based nanolaminate decreases nearly twice. The results suggest that the La /Al nanolaminates deposited by ALD using H₂O as oxidant still suffer from moisture absorption of La due to the residual hydroxyl/hydrogen groups, which is harmful to obtaining the good properties of La /Al nanolaminates after annealing.

Fig. 3.

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	Fig. 3. (color online) AFM surface images of La /Al nanolaminates deposited by ALD using different oxidants.

In order to evaluate the oxidant effects on the properties of La /Al nanolaminates before and after annealing, the valence band offset (VBO) is determined by using the method proposed by Kraut^[16]

Fig. 4.

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	Fig. 4. (color online) The XPS core-level spectrum of Si for bulk clean Si.

Figure 5 shows the XPS core-level spectra of Si and Al for La /Al nanolaminates deposited by ALD using different oxidants. The core level energies are obtained by curve fitting to ensure high accuracy binding energy of the peak, and the spectra are referenced to the C peak at 284.8 eV. For the H₂O-based ALD process, the energy difference between the Al and VBM decreases from 71.79 eV to 71.15 eV after annealing as shown in Fig. 5(a). The energy differences between Al and Si are 23.40 eV and 23.59 eV for the as-deposited and the annealed thin nanolaminates, respectively, as shown in Fig. 5(b). Using these values with Eq. (1), we obtain a VBO of 3.71 eV and 4.16 eV for the as-deposited and the annealed La /Al nanolaminates, respectively. The VBO value increases 0.45 eV after annealing which may be related to the vacancies generated due to the moisture absorption of La . For the O₃-based ALD process, the energy difference between the Al and VBM decreases from 71.79 eV to 71.62 eV after annealing, as shown in Fig. 5(c). The energy difference between Al and Si was 24.52 eV and 24.62 eV for the as-deposited and the annealed thin nanolaminates, respectively, as shown in Fig. 5(d). Substituting these values into Eq. (1), the values of VBO of 2.59 eV and 2.66 eV are obtained for the as-deposited and the annealed La /Al nanolaminates, respectively. The negligible variation of VBO value for O₃-based La /Al nanolaminates is observed after annealing.

Fig. 5.

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	Fig. 5. (color online) The XPS core-level spectra of Si and Al for 10-nm and 1-nm La /Al nanolaminates before annealing (As-dep.) and after annealing (being annealed).

The band offset change after annealing should result in a shift in V .^[17,18] Figure 6 shows the C–V characteristics of 5-nm La /Al nanolaminates deposited by ALD with using different oxidants before and after annealing (f = 100 kHz). The gate voltage ( is swept from accumulation to inversion and then swept back. For the H₂O-based ALD process as shown in Fig. 6(a), the hysteresis in the C–V curve increases significantly and the capacitance value of La /Al nanolaminates decreases after annealing. It can be explained by the fact that the moisture absorption of La deteriorates the properties of nanolaminates, such as the increase of nanolaminate roughness, degradation of density and formation of La(OH)₃ with a low permittivity.^[15] Furthermore, the C–V curve shifts from the positive to the negative direction after annealing, showing that the positive charge increases after annealing, which is associated with the generation of oxygen vacancies.^[19,20] The permittivity obtained for H₂O-based La /Al nanolaminate is 9.6 and it decreases to 6.7 after annealing. For the O₃-based ALD process, due to the densification of La /Al nanolaminates and decrease of the impurity concentration, the C–V characteristics of La /Al nanolaminates improve after annealing as shown in Fig. 6(b). Furthermore, the capacitance value decreases after annealing, which may be attributed to the low dielectric interfacial layer thickness between La /Al nanolaminates and Si substrate increasing. Moreover, a hump appears in the C–V curve and it disappears after thermal annealing process. It can be explained by the fact that the traps or defects are generated in the nanolaminate due to the strong oxidization and lability of O₃.^[21] Thermal annealing supplies enough thermal energy for the decomposition and re-composition of traps or defects, the traps or defects decrease and the hump disappears after annealing. The permittivities obtained for the as-deposited and annealed O₃-based La /Al nanolaminate are 14.4 and 13.7, respectively.

Fig. 6.

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	Fig. 6. (color online)The C–V characteristics of La /Al nanolaminates deposited by ALD with different oxidants before and after annealing (f = 100 kHz).

Breakdown characteristics of the Al/5 nm La /Al nanolaminate/p-type Si/Al capacitor structure are shown in Fig. 7. The capacitor with H₂O-based La /Al nanolaminate shows considerably low breakdown voltage due to the moisture absorption deteriorating the properties of nanolaminates. A high breakdown voltage of 7.54 V for the capacitor with O₃-based La /Al nanolaminate is measured at a leakage current of 1 mA while that of the H₂O-based La /Al nanolaminate is 3.03 V. The breakdown voltage improvement results from the suppressed moisture absorption, which is consistent with the C–V measurement results.

Fig. 7.

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	Fig. 7. (color online) Breakdown characteristics of Al/La /Al nanolaminates/Si MIS capacitors with different oxidants.

Preliminary results in the development of ALD-grown La /Al nanolaminates for serving as dielectric are presented. The oxidants’ (H₂O and O effects on the physical properties and electrical characteristics of La /Al nanolaminates are investigated. The XPS results indicate that compared with the O₃-based process, La /Al nanolaminates grown with H₂O used as an oxidant suffer the moisture absorption of La after thermal annealing due to the relatively high residue level of hydroxyl/hydrogen groups. As a result, a large valence band offset is observed and the surface roughnesses of La /Al nanolaminates increase significantly due to the formation of low density hexagonal La(OH)₃. Further electrical measurement shows that the electrical characteristics of H₂O-based La /Al nanolaminates are deteriorated and low permittivity and breakdown voltage are achieved. All results indicate that O₃ is a more appropriate oxidant to deposit La /Al nanolaminates for electronic device applications.