Background

According to Moore’s law, gate dielectrics applied in complementary metal oxide semiconductor (CMOS) devices with an equivalent oxide thickness (EOT) of no more than 1 nm are needed since the 45-nm technology node. Consequently, insulating materials with much higher dielectric constant than that of silicon oxide or oxynitrides are required to gain an acceptable gate leakage current and static power consumption [1]. Due to its appreciably high dielectric constant (20 ~ 25), large band gap (Eg > 5 eV), and valence band offset (VBO > 1 eV) relative to silicon, lanthanum aluminate (LaAlO3) has been considered as one of the alternative materials to replace SiO2 as the insulator [2, 3]. Benefit from its growth mechanism controlled by a self-limited surface reaction, atomic layer deposition (ALD) is being considered as a promising deposition technique to produce high quality high-k thin films with excellent conformality and precise thickness controllability [4]. However, ALD is a low-temperature deposition technique, thus high temperature post-deposition annealing (PDA) is needed to eliminate trapped charges and dangling bonds in high-k dielectric films after the deposition process [5, 6]. In addition, the annealing treatment can also be of help to reduce interface trap density (D it) at the insulator/semiconductor interfaces [7]. Unfortunately, PDA process can significantly increase the thickness of interfacial layer (IL) between high-k dielectric and Si substrate by the interdiffusion of the dielectric and silicon, resulting in the decrease of dielectric constant for insulators [8]. Besides, it has been reported that different annealing treatments affect high-k films and interfaces both structurally and electrically in varying degrees [9]. The interfacial properties, including the amount of oxide-trapped charges, fixed oxide charges, interface traps, oxygen vacancies, and dangling bonds, play an important role in determining the electrical characteristics of dielectric film [10].

In this paper, the effects of different annealing ambients on the physical and electrical characteristics of LaAlO3 films grown on p-type Si substrate by ALD technique were investigated. Post-grown rapid thermal annealing was carried out at 600 °C for 1 min in vacuum, N2, and O2, respectively. Among, attentions were focused on the interfacial properties of LaAlO3/Si structures to analyze the effects of different annealing ambients.

Methods

LaAlO3 dielectric films were deposited on p-type Si (100) wafers by the Picosun R-150 atomic layer deposition reactor. Prior to the deposition, the wafers were treated with a 1:50 diluted HF solution to remove the native SiO2, followed by a 60-s rinse in demonized water. Under the deposition temperature of 300 °C, La(i−PrCp)3 and TMA were used as the La and Al precursors, while O3 was used as the oxygen source. Setting the pulse ratio of La and Al precursor as 3:1, the La:Al stoichiometric ratio of the deposited films is approximately 1:1 [11]. By varying the number of ALD cycles, LaAlO3 films with the thickness of ~4 and ~10 nm were prepared. After the deposition of LaAlO3 films, post-grown rapid thermal annealing (RTA) was carried out immediately at 600 °C for 1 min in vacuum, N2, and O2 ambients, respectively. The film thickness was measured by Woollam M2000D spectroscopic ellipsometry (SE). The microstructures of the gate insulators (LaAlO3 dielectric) were observed by cross-sectional high resolution transmission electron microscopy (HRTEM) performed with the [100] direction [12] of the Si substrate. The chemical composition of the fabricated films was examined by time of flight secondary ion mass spectrometry (TOF-SIMS). The band structures of the films were examined by the X-ray photoelectron spectroscopy (XPS) measurements. All the wafers were etched by Ar+ for 10 s (0.26 nm/s) to remove the impurities on the film surface. In this experiment, the 10-nm LaAlO3 film was used to obtain the XPS spectra for thick amorphous LaAlO3, and the 4-nm LaAlO3/Si structure was thin enough to obtain XPS spectra from both the LaAlO3 film and the underlying silicon substrate. The electrical properties of the 4-nm LaAlO3 films were measured using a metal-insulator semiconductor (MIS) capacitor structure. The MIS capacitors were fabricated by magnetron sputtering 150 nm Pt on the surface of the wafers through a shadow mask (metal gate with a diameter of 300 μm). The electrical properties, including capacitance-voltage (C-V), conductance-voltage (G-V), and leakage current density-voltage (J-V) characteristics, were measured using an Agilent B1500A analyzer.

For simplicity, the as-grown and annealed films in vacuum, N2, and O2 ambients were assigned as S1, S2, S3, and S4, respectively.

Results and Discussion

As shown in Fig. 1, O 1s XPS spectrums for the ~4-nm as-grown and annealed LaAlO3 films in vacuum, N2, and O2 ambients were analyzed to investigate the chemical bonding states near the interface between the LaAlO3 films and Si substrate. The peak position of C 1s at 284.6 eV was used as the calibration reference. The O 1s core level spectra consists of five peaks, which are approximately at 529.0 eV (I), 530.7 eV (II), 531.6 eV (III), 532.2 eV (IV), and 532.8 eV (V). These peaks correspond to the chemical bonds of La–O–La, La–O–Al, Al–O–Al, La–O–H, and Si–O–Si, respectively [13, 14]. Among the five peaks, peak I and peak III come from chemical products La2O3 and Al2O3 in the deposition process. The high temperature annealing process promotes the fracture and recombination of chemical bonds, as a result, the amount of Al–O–Al decreases while the amount of La–O–Al increases. After the annealing treatments, slight variation for the La–O–La signal was observed since few La–O–La bonds were formed in the deposition process due to the high formation enthalpy of La2O3 [15]. Peak IV, related to La(OH) x , may come from the hygroscopicity of the La2O3 [16]. After high temperature annealing treatments, significant decrease in the intensity of peak IV could be observed, indicating the reduction of hydroxyl groups [17]. It is found that the intensity of Peak V shows a more obvious increase when the annealing treatment was carried out in O2 ambient compared with that in N2 or vacuum ambient, indicating that more Si–O–Si bonds were formed during the RTA process in O2 ambient. The Si–O–Si bonds are considered to come from SiO x , which is a main component of IL between the LaAlO3 film and silicon substrate [18]. So, it can be concluded that the formation of IL was enhanced during the RTA process, especially in O2 ambient.

Fig. 1
figure 1

O 1s XPS spectra of ~4-nm LaAlO3 films S1 ~ S4

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To further investigate the structural information at the dielectric/Si interface, cross-sectional HRTEM analyses for S1 and S4 are shown in Fig. 2. Both S1 and S4 exhibit an amorphous structure as no nanometer-sized crystal or long-range ordered crystal region was observed [19]. Compared with Fig. 2a, a much thicker amorphous transition region about ~2.7 nm between the deposited film and Si substrate is observed in Fig. 2b, indicating a much thicker IL formation during the annealing process in O2 ambient. We attribute this difference of IL formation to the RTA-induced interdiffusion of LaAlO3 films and silicon substrates. In order to address the evolution of the chemical composition at the LaAlO3/Si interface and within the LaAlO3 films, TOF-SIMS depth profiles of Si+, La+, SiO3 −, and OH− clusters were acquired on S1 and S4, as shown in Fig. 3. The intensity of the signals was dealt with normalization method, and depth values were calibrated by HRTEM results. As shown in the depth profiles of Si+ and La+, during the annealing process, substrate Si atoms diffuse into the upper LaAlO3 film, and the diffusion of La atoms in the opposite direction occurs simultaneously. HRTEM analysis reveals the existence of a thicker IL in S4, and now this result can be further confirmed from the intensity of SiO3 − signals which suggest the extra presence of a SiO x -like component coexisting with the La-based profile (La+) in the region at the nanolaminate/substrate interface for S4. Besides, compared with S1, the OH− profile is reduced after annealing treatment in O2 ambient, in good agreement with the XPS results.

Fig. 2
figure 2

Cross-sectional HRTEM images of ~4-nm fabricated LaAlO3 samples. a S1. b S4

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Fig. 3
figure 3

TOF-SIMS depth profiles of ~4-nm fabricated LaAlO3 samples. a S1. b S4

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Figure 4 shows the C-V and G-V characteristics of the fabricated MIS capacitors using the as-grown and annealed LaAlO3 films as insulators. C-V characteristics were obtained by sweeping forward (bias from negative to positive) and backward (bias from positive to negative) at the frequency of 100 kHz. G-V curves were obtained simultaneously with the C-V curves measured with applied voltage sweeping from positive to negative. The accumulation capacitance values of the MIS capacitors using the fabricated LaAlO3 films S1 ~ S4 as insulators were obtained to be 1.28, 1.20, 1.10, and 0.93 μF/cm2, respectively. The annealing treatment of LaAlO3 films in different ambients results in varying degrees of decrease in accumulation capacitance. In accordance with the XPS results shown in Fig. 1 and TOF-SIMS results shown in Fig. 3, such decreases in the accumulation capacitance are attributed to the formation of lower dielectric constant ILs, which primarily consist of SiO x -like component and La-silicate, due to the interdiffusion of LaAlO3 films and silicon substrates during the RTA treatment [20].

Fig. 4
figure 4

C-V and G-V characteristics for the fabricated MIS capacitors using 4-nm S1 ~ S4 as insulators. a S1. b S2. c S3. d S4. The capacitors were measured at the frequency of 100 kHz

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The flat band voltages (V FB) of the capacitors were extracted from the simulation software named Hauser NCSU CVC program taking into account of quantum-mechanical effects [21]. Considering the work function difference between the p-type Si substrate and Pt electrode, the ideal V FB should be 0.73 V. However, the actual V FB swept backward for the as-grown LaAlO3 film is 0.01 V, indicating the existence of effective positive oxide charges in the LaAlO3 film, which may be attributed to the existence of positive fixed oxide charges and oxide-trapped charges. Compared with the as-grown LaAlO3 film, positive V FB shifts for the LaAlO3 films annealed at 600 °C in vacuum, N2, and O2 ambients were observed to be 0.06, 0.33, and 0.51 V, respectively, revealing the reduction of positive oxide charges during the RTA treatments [22]. Assuming the two-dimensional distribution of traps in the vicinity of the interface contributing to the film capacitance, we investigated the charge trapping behavior through the C-V hysteresis characteristics. The trapped charge density (N ot) can be estimated following the equations [23, 24]:

$$ {C}_{\mathrm{ox}}={C}_{\mathrm{ac}}\left[1+\left(\frac{G_{\mathrm{ac}}}{\omega {C}_{\mathrm{ac}}}\right)\right] $$
(1)
$$ {N}_{\mathrm{ot}}=\frac{\varDelta {V}_{\mathrm{FB}}{C}_{\mathrm{ox}}}{qA} $$
(2)

Where C ox is the gate oxide capacitance, C ac is the measured accumulation capacitance, G ac is the conductance in accumulation region, q is the electron charge (1.602 × 1019 C), A is the electrode area, and ω is the angular frequency. The hysteresis width (ΔV FB) of S1 ~ S4 were extracted to be 299, 135, 122, and 72 mV, separately. Thus, using Eqs. (1) and (2), the N ot values of S1 ~ S4 were determined to be 2.47 × 1012, 1.03 × 1012, 8.47 × 1011, and 4.20 × 1011 cm−2, respectively. As expected, a visible decrease in N ot could be observed after annealing treatments, indicating that the reduction of the oxide trapped charges, which may be attributed to the existence of oxygen vacancies, should be one of the causes leading to the positive shifts of V FB. In addition, the larger decrease in the magnitude of N ot for the LaAlO3 film annealed in O2 ambient may be owing to the further reduction of oxygen vacancies during the oxygen atmosphere annealing process [25].

Moreover, as shown in Fig. 4, varying degrees of humps in the C-V curves could be observed, which may be caused by the existence of interfacial traps [26, 27]. Compared with Fig. 4a, it can be seen that the humps were reduced after the annealing treatments, especially in O2 ambient. Considering this, the values of D it for the fabricated MIS capacitors extracted from the Hill-Coleman single-frequency approximation were discussed, and the results are shown in Table 1. The D it values for the fabricated MIS capacitors using S1 ~ S4 as insulator are about 9.65 × 1012, 5.12 × 1012, 4.29 × 1012, and 2.50 × 1012 eV−1cm−2, respectively. After the annealing treatments in different ambients, varying degrees of decrease in the values of D it are observed, agreeing with the variation trend of the humps in the C-V curves. This phenomenon can be attributed to the decrease of defects and dangling bonds near the interface during the RTA process [28, 29].

Table 1 Various parameters for the fabricated MIS capacitors using S1 ~ S4 as insulators
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To further investigate the interfacial properties between the LaAlO3 films and Si substrate, the VBOs of LaAlO3/Si structures were analyzed by XPS measurements. The VBOs of LaAlO3 films relative to Si substrate were determined by a core level photoemission-based method similar to that of Kraut et al [30, 31] as illustrated in Fig. 5a. Accordingly, the VBO is given by Eq. (3):

Fig. 5
figure 5

Band alignments of LaAlO3/Si structures. a Schematic of band energy alignment diagram for a LaAlO3/Si structure; XPS core level spectra of b Si 2p and valence band for bulk clean silicon, c Al 2p and valence band for 10-nm LaAlO3 films, and d Si 2p and Al 2p for 4-nm LaAlO3 films on p-Si(100)

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$$ \varDelta {E}_{\mathrm{v}}={\left({E}_{\mathrm{Si}\ 2 p}-{E}_{\mathrm{V}}\right)}_{\mathrm{Si}}-{\left({E}_{\mathrm{Al}\ 2 p}-{E}_V\right)}_{{\mathrm{Thick}\ \mathrm{LaAlO}}_3}-{\left({E}_{\mathrm{Si}\ 2 p}-{E}_{\mathrm{Al}\ 2 p}\right)}_{{\mathrm{LaAlO}}_3/\mathrm{Si}} $$
(3)

Where ESi 2p is the binding energy of Si 2p shallow core level and EAl 2p is the binding energy of Al 2p shallow core level. Valence band maximum (Ev) is the binding energy corresponding to the top of the valence band (VB) for Si and LaAlO3, respectively. The positions of the Ev for both Si and dielectrics were determined by linearly extrapolating the segment of maximum negative slope to the background level [32].

Figure 5b, c shows the shallow core-level and VB spectra for bulk clean p-type Si(100) and thick 10-nm LaAlO3 films, while Fig. 5d shows the shallow core-level spectrums for 4-nm LaAlO3/Si structures. The energy difference for bulk p-type Si (100) between the XPS spectra of Si 2p and Ev was determined to be 98.9 ± 0.05 eV. Therefore, according to Eq. (3), the VBOs of as-grown and annealed LaAlO3 films in vacuum, N2, and O2 ambients relative to p-type Si substrate were measured to be 3.24 ± 0.1, 3.36 ± 0.1, 3.46 ± 0.1, and 3.55 ± 0.1 eV, respectively. It is found that the VBO values of the LaAlO3 films after annealing are obviously larger than that of the as-grown LaAlO3 film, and the largest VBO value was obtained in the O2 case. The augment of the VBO values after annealing treatments is believed to benefit from the formation of SiO x -like IL, which has much larger band offsets relative to silicon than that of LaAlO3.

Figure 6 displays the leakage current density as a function of the applied electrical field of the films with the Pt/4-nm LaAlO3/p-type Si capacitor structures. The leakage current density for the as-grown LaAlO3 film was determined to be ~7.14 × 10−4 A/cm2 at the applied electrical field of −5 MV/cm. After being annealed, at the same applied electrical field, the leakage current density values of the fabricated MIS capacitors using LaAlO3 films annealed in vacuum, N2, and O2 ambients as insulators were measured to be ~1.86 × 10−4, ~8.78 × 10−5, ~3.18 × 10−5 A/cm2, respectively. Significant decrease in the gate leakage current was observed after being annealed, especially for the O2 case, in which a decrease of more than one order of magnitude was obtained. Such a decrease of leakage current density may be primarily attributed to the change of valence band offsets at the nanolaminate/Si interface during the high temperature annealing process. It has been reported that the gate leakage current for high-k dielectric depends exponentially on potential barriers, which vary with band offsets [33]. As mentioned above, among the as-grown and annealed samples in vacuum, N2, and O2 ambients, the largest VBO value was obtained in O2-annealed LaAlO3/Si structure, providing an effective potential barrier to weaken the tunneling effect of electrons and holes in the MIS capacitor, resulting in lowest gate leakage current. In addition, the annealing treatment in O2 ambient seems to serve as a most effective way to reduce oxygen vacancies, which may also give an explanation to the significant decrease of gate leakage current in S4.

Fig. 6
figure 6

J-V characteristic of the fabricated MIS capacitors using 4-nm S1 ~ S4 as insulators

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Conclusions

In this paper, the effects of different annealing ambients on the physical and electrical properties of LaAlO3 films grown by ALD were analyzed. It was found that the amount of hydroxyl groups decreased after annealing treatments. In addition, ILs are formed after annealing treatments, resulting in the decrease of accumulation capacitance values for LaAlO3 films, especially in O2 ambient. Compared with the ideal V FB, the actual V FB value for the as-grown LaAlO3 dielectric was negatively shifted, indicating the existence of positive oxide charges. After RTA treatments in different ambients, oxygen vacancies and defects were reduced, resulting in positive V FB shifts in varying degrees. Significant decrease in the leakage current density was found when the LaAlO3 films were annealing treated, especially for the LaAlO3 film annealed in O2 ambient, in which a decrease of more than one order of magnitude was found. Such a decrease in the leakage current density may be primarily attributed to the larger values of valence band offsets and the reduction of oxygen vacancies near the LaAlO3/Si interface.