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Ni doping significantly improves dielectric properties of La2O3 films
Journal Pre-proof Ni doping significantly improves dielectric properties of La2O3 films Shuan Li, Youyu Lin, Yong wu, Yanqing Wu, Xingguo Li, Wenhuai Tian PII:
S0925-8388(19)34715-2
DOI:
https://doi.org/10.1016/j.jallcom.2019.153469
Reference:
JALCOM 153469
To appear in:
Journal of Alloys and Compounds
Received Date: 4 November 2019 Revised Date:
16 December 2019
Accepted Date: 19 December 2019
Please cite this article as: S. Li, Y. Lin, Y. wu, Y. Wu, X. Li, W. Tian, Ni doping significantly improves dielectric properties of La2O3 films, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153469. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Shuan Li: Conceptualization, Methodology, Validation, Investigation, Writing - Original Draft Youyu Lin: Validation, Investigation, Software Yong wua: Investigation, Formal analysis Yanqing Wu: Conceptualization, Supervision Xingguo Li: Resources, Writing - Review & Editing, Supervision, Funding acquisition Wenhuai Tian: Resources, Writing - Review & Editing, Project administration
Ni doping significantly improves dielectric properties of La2O3 films Shuan Li a,b, Youyu Lina, Yong wua, Yanqing Wua, Xingguo Li a,* and Wenhuai Tian b, ** a
Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China b
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, 100083, China
Abstract With rapid development of integrated circuits, a long-standing challenge is to seek new gate dielectrics to replace HfO2. Lanthanum oxide (La2O3) has earned more and more attention with its attractive performance. Rational doping is an extremely effective way to further improve its dielectric properties. In this work, Ni is first doped into La2O3 by reactive co-sputtering for a novel dielectric film. We demonstrate that proper amount of Ni (~10.04%) doping can effectively improve the performance of La2O3, exhibiting desired microstructure, large band gap (5.7 eV), suitable band offsets (VB=2.15 eV, CB=2.43 eV) and excellent electrical properties (k=22.08). Furthermore, we optimized the performance of Ni-doped La2O3 (LNO) films by altering annealing temperature. The results suggest that 600 °C is the most suitable annealing temperature for LNO films, leading to a lower leakage current density of 2.06 ×10-4 A/cm2. Our work provides a new insight to select the suitable element to modify rare earth oxides for next-generation gate dielectrics. Keywords： ：Rare earth; High k dielectric; Oxides; Nickel; Sputtering 1. Introduction
Fig. 1. Status of metal doped modified rare earth oxides.
As we known, silicon-based complementary metal oxide semiconductor (CMOS) field effect transistors (FETs) are the most critical electronic devices in integrated circuits, exhibiting low power dissipation and high interference resistance [1-4]. With the feature sizes of CMOSFETs shrinking, the thickness of gate dielectric (SiO2) reaches its physical limit, resulting in an * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X.G. Li), [email protected] (W.H. Tian).
unacceptable consumption [5]. To deal with this issue, scientists put forward high k materials to be gate dielectrics [6, 7]. In the past decades, HfO2 and Hf-based high k oxides have gained much attention [8-10]. Nevertheless, HfO2 has low crystallization temperature (<500 ), which would lead to threshold voltage instability, defects generation and huge leakage current density [11-13]. Therefore, for the long-term development of Moore's Law [14], it is indispensable to find new high k gate dielectrics to replace HfO2 [15, 16]. Recently, rare earth oxides (RE2O3) have been attracted more and more attention on account of relatively high dielectric constant and band gaps, as well as excellent thermal and chemical stability [17, 18]. However, the k value of binary rare earth oxides is similar to HfO2 [19, 20]. Fortunately, doping is an effective way to improve the dielectric properties of rare earth oxides. As shown in Fig.1, all studies involving doped modified RE2O3 for high k dielectrics can be divided into three categories, there are (1) Hf or Zr doped RE2O3 films [21, 22], (2) RE2O3 doped another RE2O3 [23, 24] and (3) conventional dielectric materials doped RE2O3, such as Ta2O5 [25], Al2O3[26] and TiO2 [27], etc. Obviously, few studies involve the doping of Fe, Ni, Mo, Cr, Cu, Co, Mn, etc. to RE2O3 to improve dielectric performance. Because it is widely believed that corresponding oxides of these metals would react with Si to form either SiO2 or a silicide, which would increase EOT (equivalent oxide thickness) and short out the field effect [28, 29]. Our previous report on Fe-doped La2O3 films conforms this point and draws that Fe is a harmful impurity for La2O3 [30]. In this work, La2O3 is selected as a gate dielectric for its large theoretical dielectric constant (~25-30), high enough band gap (>5.5 eV) and higher crystallization temperature (>500 °C) than HfO2 [18]. Ni is first incorporated into La2O3 to improve its dielectric performance. We focus on the effects of Ni content in La2O3 films and post annealing temperature on microstructure, optical properties and electrical properties. The results are exciting and can be concluded that ~10.04% Ni doping can significantly improve dielectric properties of La2O3 films. And we determine that the optimal annealing temperature is 600 °C. Besides, some corresponding explanations are given in our work. 2. Experimental details 2.1. LNO films deposition As shown in Fig.2, LNO thin films were prepared by radio frequency (RF) co-sputtering pure La target (99.95%) and pure Ni target (99.95%). In order to obtain LNO films with different Ni content, the sputtering power of the La target was kept at 90 W and the sputtering power of the Ni target was set to 0, 5, 10, 15 and 20 W, respectively. The vacuum of the chamber is pumped to 5×10-4 Pa before depositing LNO films. During sputtering, pure Ar (30 sccm) and O2 (10 sccm) were introduced into chamber to ensure that La, Ni and O atoms can react fully to form LNO films. And the sputtering pressure was maintained at 0.6-0.8 Pa. During the sputtering, the rotation speed of substrate was maintained at 20 r/min to ensure a uniform LNO film. For the convenience of discussion below, LNO films with different Ni content are labeled as LNO-W (W represents sputtering power, W=0, 5, 10, 15, 20).
2
Fig. 2. Schematic diagram of magnetron co-sputtering LNO film with La and Ni targets.
2.2. Pt/LNO/Si/Pt MOS capacitors fabrication All electrical performance tests are based on MOS capacitors (see Fig.3 (a)). As for MOS capacitors fabrication, silicon wafers (p-type (100), 1-10 Ω·cm) were selected as substrates. Prior to sputtering, the wafers were cut to the size of 10×10 mm and cleaned by Shiraki’s method [31]. Then ~17 nm LNO films were deposited on Si substrates for electrical measurement. Finally, as shown in Fig.3 (c), Pt electrodes with the thickness of 390 nm were deposited on the top of LNO film and the back of Si substrate by a pure Pt target (99.99%). The DC sputtering power of Pt target was set to 80 W meanwhile the flow rate of Ar was kept at 40 sccm. And the area of Pt electrodes (3.14×10−2 cm2) was determined by the area of a shadow mask hole. The physical picture of shadow masks and MOS capacitors is shown in Fig. 3 (b). Since there are many defects in LNO film directly deposited, one-step rapid annealing is the most convenient and effective method for removing defects. Therefore, in this work, an annealing process in pure O2 (100 sccm) was carried out to ensure excellent film quality. It should be emphasized that annealing temperature is the most important parameter. We first annealed all the samples at 500 °C and then optimize the annealing temperature in Section 3.4.
Fig. 3. Schematic diagram of (a) Pt/LNO/Si/Pt MOS capacitors, (b) the photos of shadow masks and MOS and (c) cross-section SEM photo of Pt electrode.
2.3. Characterization The characterization of LNO samples are relied on XRD, AFM, SEM, TEM, XPS, precision LCR meter and electrochemical workstation. Introduction of these devices are given in Electronic 3
Supplementary Information (ESI) in detail. 3. Results and discussion 3.1. Microstructure analysis
Fig. 4. (a) Histogram of atomic percentage of Ni/(Ni+La) as a function of Ni target sputtering power. (b) Phase diagram of La2O3 and NiO. (c) XRD patterns and (d-g) AFM images of LNO-5, LNO-10, LNO-15, LNO-20 films.
Fig. 4(a) shows the atomic ratio of Ni/(Ni+La) in LNO films with various Ni sputtering power. The atomic ratio of Ni/(Ni+La) is 7.34, 10.04, 14.15 and 17.6% for LNO-5, LNO-10, LNO-15 and LNO-20 respectively. According to the phase diagram of La2O3 and NiO (see Fig.4(b)) [32], the phases of all LNO films prepared in this work are La2O3+La2NiO4. To further investigate the microstructure of LNO films, on the one hand, XRD patterns of all LNO films are tested. The results show that all LNO films are amorphous, indicating that proper amount (<17.6%) of Ni doping has no obvious effect on the crystallization temperature of La2O3 [33]. The amorphous film is an ideal high k film state, because grain boundary can induce large leakage current density [5]. On the other hand, AFM photos are taken to characterize the surface morphology of LNO films. During AFM measurement, the contact mode is adopted (scanning range is 5×5µm, 4
frequency is 1Hz). Comparing the AFM photos in Fig. 4(d-g), the surface of LNO-10 film is flatter. With the Ni content increasing, island-like protrusions appear on the surface of LNO film, which may be ascribed the effect of Ni doping on the growth state of La2O3 films. [34]. The values of surface roughness (Ra) of LNO-5, LNO-10, LNO-15 and LNO-20 samples are 0.416, 0.118, 0.201 and 0.493 nm, respectively. Comparing Ra value of pure La2O3 film (Ra=0.166 nm) reported in our previous work [30], the surface roughness of LNO-10 films is as low as 0.118 nm and even lower than that of pure La2O3, which would ensure a small leakage current density.
Fig. 5. XPS spectra of (a) O 1s, (c) La 3d, (d) Ni 3p and (e) Si 2s peaks for LNO films. Semiquantitative analyses of (b) oxygen component and (f) Si component for LNO films.
Fig.5(a) reveals the O1s XPS spectrums of LNO films, which can be divided into three peaks of 530.1, 531.6 and 532.4 eV, corresponding to M-O bonds, oxygen vacancies (Vo) and hydroxyl or water molecules (M-OH bonds) adsorbed on the surface of the films [35, 36]. The O1s spectrums of LNO-5, LNO-10, LNO-15 and LNO-20 films were peaked and the proportions of each fraction were summarized in histograms of Fig.5(b). Obviously, with the sputtering power of Ni target increasing from 5 to 20 W, the ratios of M-O bonds in LNO film are 20.45, 34.3, 27.66 and 18.84%, respectively, which shows the trend of increasing first and then decreasing. While the trends of Vo and M-OH are opposite to M-O bonds, the ratios of Vo in LNO films are 61.36, 52.15, 52.55and 55.5% and the ratios of M-OH bonds are18.19,13.55,19.79 and 25.66%, respectively. It can be seen that LNO-10 film has the best film quality among four samples with the largest number of M-O bonds, the least number of oxygen vacancies and M-OH bonds, which gives it enough potential to enjoy the best electrical performance. Fig.5(c) shows La 3d XPS spectrums of LNO films. The main peak is composed of four sub-peaks, because the characteristic peaks of the metal La 3d are double peaks. As for the LNO-5 5
sample, the position of the characteristic peak of La 3d5/2 is 834.9 eV, which belongs to La3+ in La2O3. With the amount of Ni incorporation increasing, the La 3d5/2 peak shifts slightly toward low binding energy, indicating that part of the electrons on Ni are transferred to La with the addition of Ni, but La still exists in trivalent [37]. Since Ni 2p peak coincides with La 3d peak [38], in this work, we examined the Ni 3p peak (see Fig.5(d)). It can be seen that with the sputtering power of Ni increasing, the intensity of Ni characteristic peak gradually increases, indicating that Ni content in LNO film increases. The Ni ions in LNO-5, LNO-10 and LNO-15 films exist in divalent forms. But most of the Ni ions in LNO-20 film exist in divalent forms, some in trivalent forms. which is somewhat different from La2O3 and NiO phase diagram. Based on the study of liang [39] et al., increasing the amount of Ni2+ can enhance LNO film’s insulation and increasing the amount of Ni3+ can enhance LNO film’s conductivity. Furthermore, when the amount of Ni2+ and Ni3+ is almost the same, the film exhibits semiconductor characteristics. Therefore, it can be predicted that the insulation of the film deteriorates and leakage current density of LNO film increases with the amount of Ni incorporation increasing. Fig 5(e) shows Si 2s XPS spectrums of LNO-0, LNO-10 and LNO-20 samples to determine the interface state between LNO dielectric film and Si substrate. Herein, we use Si 2s to replace Si 2p XPS spectrums because Si 2p spectrum region overlaps with La 4d. According to previous report [40], the peak signaled at 151 eV is attributed to Si-Si bonds from Si substrate and the peak located at 153.8 eV is attributed to Si-O bonds from interfacial layer. Besides, Si 2s XPS spectrums with energy below 151 eV indicate the metallic nature of Si, for instance, La-Si bonds are reported at both location of 145.75 and 148.75 eV [40]. Obviously, no peaks can be divided lower than 151 eV in this work. Therefore, we focus on the effects of Ni doping content on Si-Si bonds and Si-O bonds. The atomic percent of Si-Si bonds and Si-O bonds extracted from the area ratio of peaks in Fig 5(e) is displayed in Fig. 5(f). With Ni doping content increasing, the atomic percent of Si-O bonds shows an increasing trend, which values are 33.27, 37.90 and 86.55%, respectively. On the contrary, the atomic percent of Si-Si bonds shows a decreasing trend, which values are 66.73, 62.10, 13.45%, respectively. This result indicated that Ni incorporation can introduce silicon oxide at the interface of Si and LNO film. Further, with Ni content increasing, the formation of low-k silicon oxide at interface tends to increase, especially for LNO-20 film, which probably leads to a decrease in dielectric constant of LNO thin film [5]. 3.2. Optical properties analysis 3.2.1 Band gap analysis The band gaps of different Ni doping LNO films can be obtained by Tauc optical method [41, 42]. As shown in Fig. 6(a), light absorption spectrums of ~200 nm LNO films deposited on quartz plates were measured. With Ni content increasing, especially for LNO-20 samples, LNO films begin to absorb light in the wavelength of 250-550 nm. Based on the curves in Fig 6(a), band gaps of LNO films can be calculated (see Fig. 6(b)). In this work, the band gap of pure La2O3 are 5.55 eV. The band gap of LNO-5 film is still 5.55 eV, indicating that the doping amount of Ni below 7.34% has little effect on band gap of La2O3 film. However, the band gap of LNO-10 film increases to 5.70 eV. According to O1s XPS results, this may be due to the fact that LNO-10 film has the best film quality and the lowest defects. As for LNO-15 and LNO-20 films, the band gaps decrease from 4.81 to 4.75 eV. With the doping amount of Ni increasing, on the one hand, the 6
quality of the film decreases, the defects in the film increase and the surface roughness increases, causing photon scattering and reducing the band gap of LNO film. On the other hand, further increasing the amount of Ni leads to the generation of impurity levels, which decreases the value of band gap [43]. Fig. 6(c) shows the variation of band gaps with Ni target sputtering power. It is clear that LNO-10 film has the largest band gap and has potential to be a gate dielectric for an insulating layer.
Fig. 6. (a) Optical absorption and (b) band gaps of LNO-W thin films (W=0, 5, 10, 15, 20). (c) The band gaps of LNO films as a function of Ni sputtering power.
3.2.2 Band offset analysis
Fig. 7. (a) VB spectra of the Si substrate and LNO-W thin films (W=0, 5, 10, 15, 20). (b) Schematic band diagram of LNO-W thin films (W=0, 5, 10, 15, 20).
7
It is essential to determine band gap offsets of high k films relative to silicon substrates. According to reports by He[44, 45] et al., the valence band (VB) offset of LNO film relative to the Si substrate can be obtained by the following equation: ∆Ev(LNO-Si)=Ev(LNO)-Ev(Si) (1) As shown in Fig. 7(a), Ev(LNO) and Ev(Si) can be determined by linear extrapolation method. Taking advantage of equation (1), the VB offsets of different Ni content doped La2O3 are derived. The VB offsets of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 films relative to Si substrate are 2.55, 2.52, 2.15, 2, and 1.9 eV, respectively. It can be seen that with the amount of Ni incorporation increasing, VB offsets of LNO films are gradually decreasing. Conductor band (CB) offsets of LNO films can be determined by the band gaps of LNO films and Si as well as ∆Ev (LNO-Si). The specific calculation is through Equation (2) [46]: ∆Ec(LNO-Si)=Eg(LNO)-∆Ev(LNO-Si)-Eg(Si) (2) where Eg(LNO)is band gap of LNO films whose value has been obtained in section 3.2.1. Eg(Si) is band gap of Si substrate (1.12 eV). The CB offsets of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 films are 1.88, 1.93, 2.43, 1.69 and 1.73 eV, respectively. As exhibited in Fig.7(b), the band gap structures of La2O3 films doped with various Ni are obtained. According to previous report [47], both VB and CB offsets of gate dielectric layer should be greater than 1 eV. In this work, all LNO films meet this requirement, especially LNO-10 sample has VB and CB offsets of more than 2 eV. 3.3. Electrical properties analysis
Fig. 8. (a) C-V, (b) I-V and (d) Nyquist plots and the equivalent circuit of Pt/ LNO-W/Si/Pt MOS capacitors (W=0, 5, 10,15, 20). (c) k and leakage current density (Jg) of LNO films as a function of Ni sputtering power.
8
Before Ni was doped into La2O3 films, pure La2O3 films with various thickness are made into MOS capacitors to determine optimal film thickness (more details see ESI). Our result shows that 17 nm-thick La2O3 film has the best electrical performance. Therefore, all LNO films are kept at a thickness of 17 nm, hoping to get optimal dielectric properties. Fig. 8(a) displays C-V curves of Pt/ LNO-W/Si/Pt MOS capacitors measured at 100 kHz. The dielectric constant (k) can be calculated from the capacitance of C-V curves accumulation region [48, 49]. In this work, the k values of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 samples are 17.47, 20.79, 22.08, 19.55 and 19.31, respectively. It is clear that LNO-10 sample exhibits the largest dielectric constant. Although Si 2s XPS result of LNO-10 film shows a slight increase in silicon oxide at interface layer compared to pure La2O3 film, the excellent films quality including the largest number of M-O bonds and the least number of oxygen vacancies and M-OH bonds in LNO-10 film still plays a leading role and guarantees its dielectric constant is the largest. Fig. 8(b) shows I-V curves of Pt/ LNO-W/Si/Pt MOS capacitors. Among these five samples, the leakage current density (Jg) of LNO-10 film is the smallest of 1.155×10-2 A/cm2, which may be caused by two reasons. First, LNO-10 film shows excellent microstructure, for instance, LNO-10 film is amorphous and its surface roughness is smallest. Second, LNO-10 sample has largest band gap (5.70 eV) and suitable VB and CB offsets (2.15 and 2.43 eV). As summarized in Fig. 8(c), the dielectric properties of the LNO-10 sample are optimal. Besides, impedance spectrums of LNO samples were measured to understand the behavior of I-V curves. The Nyquist plots of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 samples are displayed in in Fig. 8(d). The shape of each impedance spectrum is a depressed semicircle. But the diameters of circles are completely different, which indicates that the resistances of Pt/ LNO-W/Si/Pt MOS capacitors are different [50]. With the help of Zview software, the equivalent circuit of these five groups samples can be described in the inset of Fig. 8(d), which is resistor Rp connected in parallel with CPE1 and then in series with Rs. Rs (~50 Ω) is attributed to the contribution of external test device and is independent of samples (as shown in Table S1). However, the values of Rp are completely different and they are contribution of internal resistance of MOS capacitors [25]. The Rp values of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 are determined to be 25416, 34605, 50159, 44051 and 17870 Ω, respectively. Similarly, the Rp value of LNO-10 sample is the largest, which is consistent with its least leakage current density.
Fig. 9. (a-c) High-resolution TEM images of LNO-10 film.
Based on above results, we can conclude that LNO-10 film has the best electrical performance for a new gate dielectric material. Therefore, high-resolution TEM images of LNO-10 film were taken for detailed analysis. As displayed in Fig. 9(a), the thickness of LNO film is 17 nm and the LNO film is very uniform. There is 4.3 nm interface layer between Si 9
substrate and LNO film, which is low-k layer and will decrease k value of whole Pt/LNO-W/Si/Pt MOS capacitor. According to Si 2s XPS results, the composition of this low-k layer is silicon oxide. The above XRD result shows LNO-10 film annealed at 500 °C is amorphous. However, TEM photos show that LNO-10 film partially crystallizes and partially remains amorphous. By analyzing the crystal structure of crystallized portion, La2O3 (011) and La2NiO4 (015) can be distinguished. La2NiO4 is a perovskite-like structure with an extremely large dielectric constant [51], which is an important factor for LNO-10 samples to exhibit high dielectric constant. Table 1. Electrical parameters of LNO-W samples extracted from C-V and I-V curves
Samples
LNO-0 LNO-5 LNO-10 LNO-15 LNO-20
Ni/(Ni+La) (%)
0 7.34 10.04 14.15 17.60
Thickness (nm)
~17
Cox (nF) 28.5753 34.0013 36.1145 31.9656 31.5798
k
17.47 20.79 22.08 19.55 19.31
Cfb (nF)
Vfb (V)
Qox
2.07 2.09 2.10 2.09 2.08
0.01 0.11 0.005 -0.05 0.15
4.49×10-12 4.67×10-12 5.71×10-12 5.41×10-12 4.09×10-12
(cm-2）
EOT (nm)
Jg (A/cm2) (@Vfb-1V)
3.80 3.19 3.00 3.39 3.43
6.48×10-2 2.24×10-2 1.15×10-2 3.19×10-2 2.53×10-2
3.4. Annealing temperature optimization of LNO film In order to further optimize the electrical properties of LNO-10 films, especially the leakage current values, we study the effects of annealing temperatures from 400 to 700 °C (Annealing process introduced in ESI). Fig. 10(a) and (b) present the C-V and I-V curves of LNO-10 samples annealed at various temperatures. With annealing temperatures increasing, C-V curves show right shift indicating that the defects in LNO-10 films are further removed. The k values and leakage current density extracted from C-V and I-V curves are displayed in Fig. 10(c). At annealing temperatures in the range of 0-500 °C, the k values of LNO-10 films exhibit an increased situation. Nevertheless, when annealing temperature exceeds 500 °C, the k values of LNO-10 films begin to decrease, which is ascribed to the generation of more low-k interface layers caused by higher temperatures. Dissimilarly, with annealing temperature increasing from 0 to 600 °C, the leakage current density shows a continuous decrease from 4.75×10-1 to 2.06×10-4 A/cm2, thanks to band gaps increasing from 5.27 to 5.77 eV (see Fig. S2). But with annealing temperature increasing to 700 °C, the leakage current density increases to 2.22×10-3 A/cm2. Although LNO-10 film annealed at 700 °C has the largest band gap of 5.79 eV (see Fig. S2), the XRD results show that LNO-10 film has crystallized at 700 °C (see Fig. S3). The grain boundary can act as a fast channel for charges, resulting in huge leakage current. In order to further study the annealing temperature effect on electrical behavior of LNO-10 films, impedance spectrums of LNO-10 based MOS capacitors are investigated (Fig.10(d)). The equivalent circuit including values of RsT (~50 Ω) are the same as mentioned in section 3.3. The Rp values of LNO-10 films annealed at 400, 500, 600 and 700 °C are achieved by fitting (as shown in Table S2). When the annealing temperature is raised from 400 to 700 °C, the values of RpT are 12165, 50159, 223500 and 171730 Ω, respectively. It can be seen that with the annealing temperature increasing, the internal resistance of LNO-10 films annealed at 600 °C is the largest, corresponding to its least leakage current density. In short, LNO-10 samples annealed at 600 °C exhibits superior performance, such as higher k value (>20) and the smallest leakage current 10
density (2.06 ×10-4 A/cm2). Such excellent performance will enable LNO films to be used as potential high k gate dielectrics.
Table 2. Electrical parameters of LNO-10 samples annealed at 0, 400, 500, 600 and 700
Samples
As-dep 400 500 600 700
Cox (nF) 14.0608 28.4443 36.1145 34.1743 30.0679
k
8.60 17.39 22.08 20.90 18.39
Cfb (nF)
Vfb (V)
Qox
1.93 2.07 2.10 2.10 2.08
-1.47 -0.29 0.005 0.18 0.28
6.35×10-12 6.17×10-12 5.71×10-12 4.21×10-12 3.11×10-12
(cm-2）
EOT (nm)
Jg (A/cm2) (@Vfb-1V)
7.71 3.81 3.00 3.17 3.61
4.75×10-1 4.48×10-2 1.15×10-2 2.06 ×10-4 2.22×10-3
Fig. 10. (a) C-V, (b) I-V and (d) Nyquist plots and the equivalent circuit of Pt/ LNO-10/Si/Pt MOS capacitors. (c) k and leakage current density (Jg) of LNO films as a function of annealing temperature.
4. Conclusion A required gate dielectric film needs to have both large dielectric constant and small leakage current density. This work provides a new method to significantly improve the dielectric performance of La2O3 films by Ni doping. We demonstrate that ~10.04% Ni doping exhibits superior performance, such as the largest dielectric constant of 22.08 and the least Jg value of 1.15×10-2 A/cm2. These excellent behaviors can be attributed to the smallest surface roughness (0.118 nm), the largest band gap (5.7 eV) and suitable VB and CB offsets. In addition, the performance of LNO-10 sample was optimized by varying annealing temperature. 600 °C is the 11
most suitable annealing temperature for LNO-10 films, resulting in the leakage current density as low as 2.06 ×10-4 A/cm2. This result will shed light on the design of rare earth oxide based high k materials for next-generation gate dielectrics. Acknowledgements The authors acknowledge National Key R&D Program of China (No. 2017YFB0405902) and NSFC (No. 51771002, 51971004, 21771006 and U1607126). References [1] I.A.Y. Mark T. Bohr, CMOS scaling trends and beyond, IEEE Micro 37 (2017) 21-29. [2] J. Robertson, R.M. Wallace, High-K materials and metal gates for CMOS applications, Mat. Sci. Eng. R 88 (2015) 1-41. [3] G.D. Robert M. Wallace and Wilk, High-k dielectrics and MOSFET characteristics Crit. Rev. Solid State Mater. Sci. 28 (2003) 231-285. [4] J.A. del Alamo, Nanometre-scale electronics with III-V compound semiconductors, Nature 479 (2011) 317-323. [5] J. Robertson, High dielectric constant oxides, Eur. Phys. J-Appl. Phys. 28 (2004) 265-291. [6] A.I. Kingon, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature 406 (2000) 1032-1038. [7] M.T. Bohr, R.S. Chau, T. Ghani, K. Mistry, The high-k solution, IEEES 44 (2007) 29 - 35. [8] Y.B. Yoo, J.H. Park, K.H. Lee, H.W. Lee, K.M. Song, S.J. Lee, H.K. Baik, Solution-processed high-k HfO2 gate dielectric processed under softening temperature of polymer substrates, J. Mater. Chem. C 1 (2013) 1651-1658. [9] K. Yamamoto, W. Deweerd, M. Aoulaiche, M. Houssa, S. De Gendt, S. Horii, M. Asai, A. Sano, S. Hayashi, M. Niwa, Electrical and physical characterization of remote plasma oxidized HfO2 gate dielectrics, IEEE Trans. Electron Devices 53 (2006) 1153-1160. [10] G. He, X. Chen, Z. Sun, Interface engineering and chemistry of Hf-based high-k dielectrics on III– V substrates, Surf. Sci. Rep. 68 (2013) 68-107. [11] J.W. Zhang, G. He, L. Zhou, H.S. Chen, X.S. Chen, X.F. Chen, B. Deng, J.G. Lv, Z.Q. Sun, Microstructure
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Ni was first incorporated into La2O3 for a novel gate dielectric. La2O3 films with different Ni doping amount are prepared by reactive sputtering. Proper amount Ni doping can improve the dielectric properties of La2O3. 10.04% Ni-doped La2O3 films show best dielectric performance. 600 °C is optimal annealing temperature for Ni-doped La2O3 films.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34715-2
DOI:
https://doi.org/10.1016/j.jallcom.2019.153469
Reference:
JALCOM 153469
To appear in:
Journal of Alloys and Compounds
Received Date: 4 November 2019 Revised Date:
16 December 2019
Accepted Date: 19 December 2019
Please cite this article as: S. Li, Y. Lin, Y. wu, Y. Wu, X. Li, W. Tian, Ni doping significantly improves dielectric properties of La2O3 films, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153469. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Shuan Li: Conceptualization, Methodology, Validation, Investigation, Writing - Original Draft Youyu Lin: Validation, Investigation, Software Yong wua: Investigation, Formal analysis Yanqing Wu: Conceptualization, Supervision Xingguo Li: Resources, Writing - Review & Editing, Supervision, Funding acquisition Wenhuai Tian: Resources, Writing - Review & Editing, Project administration
Ni doping significantly improves dielectric properties of La2O3 films Shuan Li a,b, Youyu Lina, Yong wua, Yanqing Wua, Xingguo Li a,* and Wenhuai Tian b, ** a
Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China b
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, 100083, China
Abstract With rapid development of integrated circuits, a long-standing challenge is to seek new gate dielectrics to replace HfO2. Lanthanum oxide (La2O3) has earned more and more attention with its attractive performance. Rational doping is an extremely effective way to further improve its dielectric properties. In this work, Ni is first doped into La2O3 by reactive co-sputtering for a novel dielectric film. We demonstrate that proper amount of Ni (~10.04%) doping can effectively improve the performance of La2O3, exhibiting desired microstructure, large band gap (5.7 eV), suitable band offsets (VB=2.15 eV, CB=2.43 eV) and excellent electrical properties (k=22.08). Furthermore, we optimized the performance of Ni-doped La2O3 (LNO) films by altering annealing temperature. The results suggest that 600 °C is the most suitable annealing temperature for LNO films, leading to a lower leakage current density of 2.06 ×10-4 A/cm2. Our work provides a new insight to select the suitable element to modify rare earth oxides for next-generation gate dielectrics. Keywords： ：Rare earth; High k dielectric; Oxides; Nickel; Sputtering 1. Introduction
Fig. 1. Status of metal doped modified rare earth oxides.
As we known, silicon-based complementary metal oxide semiconductor (CMOS) field effect transistors (FETs) are the most critical electronic devices in integrated circuits, exhibiting low power dissipation and high interference resistance [1-4]. With the feature sizes of CMOSFETs shrinking, the thickness of gate dielectric (SiO2) reaches its physical limit, resulting in an * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X.G. Li), [email protected] (W.H. Tian).
unacceptable consumption [5]. To deal with this issue, scientists put forward high k materials to be gate dielectrics [6, 7]. In the past decades, HfO2 and Hf-based high k oxides have gained much attention [8-10]. Nevertheless, HfO2 has low crystallization temperature (<500 ), which would lead to threshold voltage instability, defects generation and huge leakage current density [11-13]. Therefore, for the long-term development of Moore's Law [14], it is indispensable to find new high k gate dielectrics to replace HfO2 [15, 16]. Recently, rare earth oxides (RE2O3) have been attracted more and more attention on account of relatively high dielectric constant and band gaps, as well as excellent thermal and chemical stability [17, 18]. However, the k value of binary rare earth oxides is similar to HfO2 [19, 20]. Fortunately, doping is an effective way to improve the dielectric properties of rare earth oxides. As shown in Fig.1, all studies involving doped modified RE2O3 for high k dielectrics can be divided into three categories, there are (1) Hf or Zr doped RE2O3 films [21, 22], (2) RE2O3 doped another RE2O3 [23, 24] and (3) conventional dielectric materials doped RE2O3, such as Ta2O5 [25], Al2O3[26] and TiO2 [27], etc. Obviously, few studies involve the doping of Fe, Ni, Mo, Cr, Cu, Co, Mn, etc. to RE2O3 to improve dielectric performance. Because it is widely believed that corresponding oxides of these metals would react with Si to form either SiO2 or a silicide, which would increase EOT (equivalent oxide thickness) and short out the field effect [28, 29]. Our previous report on Fe-doped La2O3 films conforms this point and draws that Fe is a harmful impurity for La2O3 [30]. In this work, La2O3 is selected as a gate dielectric for its large theoretical dielectric constant (~25-30), high enough band gap (>5.5 eV) and higher crystallization temperature (>500 °C) than HfO2 [18]. Ni is first incorporated into La2O3 to improve its dielectric performance. We focus on the effects of Ni content in La2O3 films and post annealing temperature on microstructure, optical properties and electrical properties. The results are exciting and can be concluded that ~10.04% Ni doping can significantly improve dielectric properties of La2O3 films. And we determine that the optimal annealing temperature is 600 °C. Besides, some corresponding explanations are given in our work. 2. Experimental details 2.1. LNO films deposition As shown in Fig.2, LNO thin films were prepared by radio frequency (RF) co-sputtering pure La target (99.95%) and pure Ni target (99.95%). In order to obtain LNO films with different Ni content, the sputtering power of the La target was kept at 90 W and the sputtering power of the Ni target was set to 0, 5, 10, 15 and 20 W, respectively. The vacuum of the chamber is pumped to 5×10-4 Pa before depositing LNO films. During sputtering, pure Ar (30 sccm) and O2 (10 sccm) were introduced into chamber to ensure that La, Ni and O atoms can react fully to form LNO films. And the sputtering pressure was maintained at 0.6-0.8 Pa. During the sputtering, the rotation speed of substrate was maintained at 20 r/min to ensure a uniform LNO film. For the convenience of discussion below, LNO films with different Ni content are labeled as LNO-W (W represents sputtering power, W=0, 5, 10, 15, 20).
2
Fig. 2. Schematic diagram of magnetron co-sputtering LNO film with La and Ni targets.
2.2. Pt/LNO/Si/Pt MOS capacitors fabrication All electrical performance tests are based on MOS capacitors (see Fig.3 (a)). As for MOS capacitors fabrication, silicon wafers (p-type (100), 1-10 Ω·cm) were selected as substrates. Prior to sputtering, the wafers were cut to the size of 10×10 mm and cleaned by Shiraki’s method [31]. Then ~17 nm LNO films were deposited on Si substrates for electrical measurement. Finally, as shown in Fig.3 (c), Pt electrodes with the thickness of 390 nm were deposited on the top of LNO film and the back of Si substrate by a pure Pt target (99.99%). The DC sputtering power of Pt target was set to 80 W meanwhile the flow rate of Ar was kept at 40 sccm. And the area of Pt electrodes (3.14×10−2 cm2) was determined by the area of a shadow mask hole. The physical picture of shadow masks and MOS capacitors is shown in Fig. 3 (b). Since there are many defects in LNO film directly deposited, one-step rapid annealing is the most convenient and effective method for removing defects. Therefore, in this work, an annealing process in pure O2 (100 sccm) was carried out to ensure excellent film quality. It should be emphasized that annealing temperature is the most important parameter. We first annealed all the samples at 500 °C and then optimize the annealing temperature in Section 3.4.
Fig. 3. Schematic diagram of (a) Pt/LNO/Si/Pt MOS capacitors, (b) the photos of shadow masks and MOS and (c) cross-section SEM photo of Pt electrode.
2.3. Characterization The characterization of LNO samples are relied on XRD, AFM, SEM, TEM, XPS, precision LCR meter and electrochemical workstation. Introduction of these devices are given in Electronic 3
Supplementary Information (ESI) in detail. 3. Results and discussion 3.1. Microstructure analysis
Fig. 4. (a) Histogram of atomic percentage of Ni/(Ni+La) as a function of Ni target sputtering power. (b) Phase diagram of La2O3 and NiO. (c) XRD patterns and (d-g) AFM images of LNO-5, LNO-10, LNO-15, LNO-20 films.
Fig. 4(a) shows the atomic ratio of Ni/(Ni+La) in LNO films with various Ni sputtering power. The atomic ratio of Ni/(Ni+La) is 7.34, 10.04, 14.15 and 17.6% for LNO-5, LNO-10, LNO-15 and LNO-20 respectively. According to the phase diagram of La2O3 and NiO (see Fig.4(b)) [32], the phases of all LNO films prepared in this work are La2O3+La2NiO4. To further investigate the microstructure of LNO films, on the one hand, XRD patterns of all LNO films are tested. The results show that all LNO films are amorphous, indicating that proper amount (<17.6%) of Ni doping has no obvious effect on the crystallization temperature of La2O3 [33]. The amorphous film is an ideal high k film state, because grain boundary can induce large leakage current density [5]. On the other hand, AFM photos are taken to characterize the surface morphology of LNO films. During AFM measurement, the contact mode is adopted (scanning range is 5×5µm, 4
frequency is 1Hz). Comparing the AFM photos in Fig. 4(d-g), the surface of LNO-10 film is flatter. With the Ni content increasing, island-like protrusions appear on the surface of LNO film, which may be ascribed the effect of Ni doping on the growth state of La2O3 films. [34]. The values of surface roughness (Ra) of LNO-5, LNO-10, LNO-15 and LNO-20 samples are 0.416, 0.118, 0.201 and 0.493 nm, respectively. Comparing Ra value of pure La2O3 film (Ra=0.166 nm) reported in our previous work [30], the surface roughness of LNO-10 films is as low as 0.118 nm and even lower than that of pure La2O3, which would ensure a small leakage current density.
Fig. 5. XPS spectra of (a) O 1s, (c) La 3d, (d) Ni 3p and (e) Si 2s peaks for LNO films. Semiquantitative analyses of (b) oxygen component and (f) Si component for LNO films.
Fig.5(a) reveals the O1s XPS spectrums of LNO films, which can be divided into three peaks of 530.1, 531.6 and 532.4 eV, corresponding to M-O bonds, oxygen vacancies (Vo) and hydroxyl or water molecules (M-OH bonds) adsorbed on the surface of the films [35, 36]. The O1s spectrums of LNO-5, LNO-10, LNO-15 and LNO-20 films were peaked and the proportions of each fraction were summarized in histograms of Fig.5(b). Obviously, with the sputtering power of Ni target increasing from 5 to 20 W, the ratios of M-O bonds in LNO film are 20.45, 34.3, 27.66 and 18.84%, respectively, which shows the trend of increasing first and then decreasing. While the trends of Vo and M-OH are opposite to M-O bonds, the ratios of Vo in LNO films are 61.36, 52.15, 52.55and 55.5% and the ratios of M-OH bonds are18.19,13.55,19.79 and 25.66%, respectively. It can be seen that LNO-10 film has the best film quality among four samples with the largest number of M-O bonds, the least number of oxygen vacancies and M-OH bonds, which gives it enough potential to enjoy the best electrical performance. Fig.5(c) shows La 3d XPS spectrums of LNO films. The main peak is composed of four sub-peaks, because the characteristic peaks of the metal La 3d are double peaks. As for the LNO-5 5
sample, the position of the characteristic peak of La 3d5/2 is 834.9 eV, which belongs to La3+ in La2O3. With the amount of Ni incorporation increasing, the La 3d5/2 peak shifts slightly toward low binding energy, indicating that part of the electrons on Ni are transferred to La with the addition of Ni, but La still exists in trivalent [37]. Since Ni 2p peak coincides with La 3d peak [38], in this work, we examined the Ni 3p peak (see Fig.5(d)). It can be seen that with the sputtering power of Ni increasing, the intensity of Ni characteristic peak gradually increases, indicating that Ni content in LNO film increases. The Ni ions in LNO-5, LNO-10 and LNO-15 films exist in divalent forms. But most of the Ni ions in LNO-20 film exist in divalent forms, some in trivalent forms. which is somewhat different from La2O3 and NiO phase diagram. Based on the study of liang [39] et al., increasing the amount of Ni2+ can enhance LNO film’s insulation and increasing the amount of Ni3+ can enhance LNO film’s conductivity. Furthermore, when the amount of Ni2+ and Ni3+ is almost the same, the film exhibits semiconductor characteristics. Therefore, it can be predicted that the insulation of the film deteriorates and leakage current density of LNO film increases with the amount of Ni incorporation increasing. Fig 5(e) shows Si 2s XPS spectrums of LNO-0, LNO-10 and LNO-20 samples to determine the interface state between LNO dielectric film and Si substrate. Herein, we use Si 2s to replace Si 2p XPS spectrums because Si 2p spectrum region overlaps with La 4d. According to previous report [40], the peak signaled at 151 eV is attributed to Si-Si bonds from Si substrate and the peak located at 153.8 eV is attributed to Si-O bonds from interfacial layer. Besides, Si 2s XPS spectrums with energy below 151 eV indicate the metallic nature of Si, for instance, La-Si bonds are reported at both location of 145.75 and 148.75 eV [40]. Obviously, no peaks can be divided lower than 151 eV in this work. Therefore, we focus on the effects of Ni doping content on Si-Si bonds and Si-O bonds. The atomic percent of Si-Si bonds and Si-O bonds extracted from the area ratio of peaks in Fig 5(e) is displayed in Fig. 5(f). With Ni doping content increasing, the atomic percent of Si-O bonds shows an increasing trend, which values are 33.27, 37.90 and 86.55%, respectively. On the contrary, the atomic percent of Si-Si bonds shows a decreasing trend, which values are 66.73, 62.10, 13.45%, respectively. This result indicated that Ni incorporation can introduce silicon oxide at the interface of Si and LNO film. Further, with Ni content increasing, the formation of low-k silicon oxide at interface tends to increase, especially for LNO-20 film, which probably leads to a decrease in dielectric constant of LNO thin film [5]. 3.2. Optical properties analysis 3.2.1 Band gap analysis The band gaps of different Ni doping LNO films can be obtained by Tauc optical method [41, 42]. As shown in Fig. 6(a), light absorption spectrums of ~200 nm LNO films deposited on quartz plates were measured. With Ni content increasing, especially for LNO-20 samples, LNO films begin to absorb light in the wavelength of 250-550 nm. Based on the curves in Fig 6(a), band gaps of LNO films can be calculated (see Fig. 6(b)). In this work, the band gap of pure La2O3 are 5.55 eV. The band gap of LNO-5 film is still 5.55 eV, indicating that the doping amount of Ni below 7.34% has little effect on band gap of La2O3 film. However, the band gap of LNO-10 film increases to 5.70 eV. According to O1s XPS results, this may be due to the fact that LNO-10 film has the best film quality and the lowest defects. As for LNO-15 and LNO-20 films, the band gaps decrease from 4.81 to 4.75 eV. With the doping amount of Ni increasing, on the one hand, the 6
quality of the film decreases, the defects in the film increase and the surface roughness increases, causing photon scattering and reducing the band gap of LNO film. On the other hand, further increasing the amount of Ni leads to the generation of impurity levels, which decreases the value of band gap [43]. Fig. 6(c) shows the variation of band gaps with Ni target sputtering power. It is clear that LNO-10 film has the largest band gap and has potential to be a gate dielectric for an insulating layer.
Fig. 6. (a) Optical absorption and (b) band gaps of LNO-W thin films (W=0, 5, 10, 15, 20). (c) The band gaps of LNO films as a function of Ni sputtering power.
3.2.2 Band offset analysis
Fig. 7. (a) VB spectra of the Si substrate and LNO-W thin films (W=0, 5, 10, 15, 20). (b) Schematic band diagram of LNO-W thin films (W=0, 5, 10, 15, 20).
7
It is essential to determine band gap offsets of high k films relative to silicon substrates. According to reports by He[44, 45] et al., the valence band (VB) offset of LNO film relative to the Si substrate can be obtained by the following equation: ∆Ev(LNO-Si)=Ev(LNO)-Ev(Si) (1) As shown in Fig. 7(a), Ev(LNO) and Ev(Si) can be determined by linear extrapolation method. Taking advantage of equation (1), the VB offsets of different Ni content doped La2O3 are derived. The VB offsets of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 films relative to Si substrate are 2.55, 2.52, 2.15, 2, and 1.9 eV, respectively. It can be seen that with the amount of Ni incorporation increasing, VB offsets of LNO films are gradually decreasing. Conductor band (CB) offsets of LNO films can be determined by the band gaps of LNO films and Si as well as ∆Ev (LNO-Si). The specific calculation is through Equation (2) [46]: ∆Ec(LNO-Si)=Eg(LNO)-∆Ev(LNO-Si)-Eg(Si) (2) where Eg(LNO)is band gap of LNO films whose value has been obtained in section 3.2.1. Eg(Si) is band gap of Si substrate (1.12 eV). The CB offsets of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 films are 1.88, 1.93, 2.43, 1.69 and 1.73 eV, respectively. As exhibited in Fig.7(b), the band gap structures of La2O3 films doped with various Ni are obtained. According to previous report [47], both VB and CB offsets of gate dielectric layer should be greater than 1 eV. In this work, all LNO films meet this requirement, especially LNO-10 sample has VB and CB offsets of more than 2 eV. 3.3. Electrical properties analysis
Fig. 8. (a) C-V, (b) I-V and (d) Nyquist plots and the equivalent circuit of Pt/ LNO-W/Si/Pt MOS capacitors (W=0, 5, 10,15, 20). (c) k and leakage current density (Jg) of LNO films as a function of Ni sputtering power.
8
Before Ni was doped into La2O3 films, pure La2O3 films with various thickness are made into MOS capacitors to determine optimal film thickness (more details see ESI). Our result shows that 17 nm-thick La2O3 film has the best electrical performance. Therefore, all LNO films are kept at a thickness of 17 nm, hoping to get optimal dielectric properties. Fig. 8(a) displays C-V curves of Pt/ LNO-W/Si/Pt MOS capacitors measured at 100 kHz. The dielectric constant (k) can be calculated from the capacitance of C-V curves accumulation region [48, 49]. In this work, the k values of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 samples are 17.47, 20.79, 22.08, 19.55 and 19.31, respectively. It is clear that LNO-10 sample exhibits the largest dielectric constant. Although Si 2s XPS result of LNO-10 film shows a slight increase in silicon oxide at interface layer compared to pure La2O3 film, the excellent films quality including the largest number of M-O bonds and the least number of oxygen vacancies and M-OH bonds in LNO-10 film still plays a leading role and guarantees its dielectric constant is the largest. Fig. 8(b) shows I-V curves of Pt/ LNO-W/Si/Pt MOS capacitors. Among these five samples, the leakage current density (Jg) of LNO-10 film is the smallest of 1.155×10-2 A/cm2, which may be caused by two reasons. First, LNO-10 film shows excellent microstructure, for instance, LNO-10 film is amorphous and its surface roughness is smallest. Second, LNO-10 sample has largest band gap (5.70 eV) and suitable VB and CB offsets (2.15 and 2.43 eV). As summarized in Fig. 8(c), the dielectric properties of the LNO-10 sample are optimal. Besides, impedance spectrums of LNO samples were measured to understand the behavior of I-V curves. The Nyquist plots of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 samples are displayed in in Fig. 8(d). The shape of each impedance spectrum is a depressed semicircle. But the diameters of circles are completely different, which indicates that the resistances of Pt/ LNO-W/Si/Pt MOS capacitors are different [50]. With the help of Zview software, the equivalent circuit of these five groups samples can be described in the inset of Fig. 8(d), which is resistor Rp connected in parallel with CPE1 and then in series with Rs. Rs (~50 Ω) is attributed to the contribution of external test device and is independent of samples (as shown in Table S1). However, the values of Rp are completely different and they are contribution of internal resistance of MOS capacitors [25]. The Rp values of LNO-0, LNO-5, LNO-10, LNO-15 and LNO-20 are determined to be 25416, 34605, 50159, 44051 and 17870 Ω, respectively. Similarly, the Rp value of LNO-10 sample is the largest, which is consistent with its least leakage current density.
Fig. 9. (a-c) High-resolution TEM images of LNO-10 film.
Based on above results, we can conclude that LNO-10 film has the best electrical performance for a new gate dielectric material. Therefore, high-resolution TEM images of LNO-10 film were taken for detailed analysis. As displayed in Fig. 9(a), the thickness of LNO film is 17 nm and the LNO film is very uniform. There is 4.3 nm interface layer between Si 9
substrate and LNO film, which is low-k layer and will decrease k value of whole Pt/LNO-W/Si/Pt MOS capacitor. According to Si 2s XPS results, the composition of this low-k layer is silicon oxide. The above XRD result shows LNO-10 film annealed at 500 °C is amorphous. However, TEM photos show that LNO-10 film partially crystallizes and partially remains amorphous. By analyzing the crystal structure of crystallized portion, La2O3 (011) and La2NiO4 (015) can be distinguished. La2NiO4 is a perovskite-like structure with an extremely large dielectric constant [51], which is an important factor for LNO-10 samples to exhibit high dielectric constant. Table 1. Electrical parameters of LNO-W samples extracted from C-V and I-V curves
Samples
LNO-0 LNO-5 LNO-10 LNO-15 LNO-20
Ni/(Ni+La) (%)
0 7.34 10.04 14.15 17.60
Thickness (nm)
~17
Cox (nF) 28.5753 34.0013 36.1145 31.9656 31.5798
k
17.47 20.79 22.08 19.55 19.31
Cfb (nF)
Vfb (V)
Qox
2.07 2.09 2.10 2.09 2.08
0.01 0.11 0.005 -0.05 0.15
4.49×10-12 4.67×10-12 5.71×10-12 5.41×10-12 4.09×10-12
(cm-2）
EOT (nm)
Jg (A/cm2) (@Vfb-1V)
3.80 3.19 3.00 3.39 3.43
6.48×10-2 2.24×10-2 1.15×10-2 3.19×10-2 2.53×10-2
3.4. Annealing temperature optimization of LNO film In order to further optimize the electrical properties of LNO-10 films, especially the leakage current values, we study the effects of annealing temperatures from 400 to 700 °C (Annealing process introduced in ESI). Fig. 10(a) and (b) present the C-V and I-V curves of LNO-10 samples annealed at various temperatures. With annealing temperatures increasing, C-V curves show right shift indicating that the defects in LNO-10 films are further removed. The k values and leakage current density extracted from C-V and I-V curves are displayed in Fig. 10(c). At annealing temperatures in the range of 0-500 °C, the k values of LNO-10 films exhibit an increased situation. Nevertheless, when annealing temperature exceeds 500 °C, the k values of LNO-10 films begin to decrease, which is ascribed to the generation of more low-k interface layers caused by higher temperatures. Dissimilarly, with annealing temperature increasing from 0 to 600 °C, the leakage current density shows a continuous decrease from 4.75×10-1 to 2.06×10-4 A/cm2, thanks to band gaps increasing from 5.27 to 5.77 eV (see Fig. S2). But with annealing temperature increasing to 700 °C, the leakage current density increases to 2.22×10-3 A/cm2. Although LNO-10 film annealed at 700 °C has the largest band gap of 5.79 eV (see Fig. S2), the XRD results show that LNO-10 film has crystallized at 700 °C (see Fig. S3). The grain boundary can act as a fast channel for charges, resulting in huge leakage current. In order to further study the annealing temperature effect on electrical behavior of LNO-10 films, impedance spectrums of LNO-10 based MOS capacitors are investigated (Fig.10(d)). The equivalent circuit including values of RsT (~50 Ω) are the same as mentioned in section 3.3. The Rp values of LNO-10 films annealed at 400, 500, 600 and 700 °C are achieved by fitting (as shown in Table S2). When the annealing temperature is raised from 400 to 700 °C, the values of RpT are 12165, 50159, 223500 and 171730 Ω, respectively. It can be seen that with the annealing temperature increasing, the internal resistance of LNO-10 films annealed at 600 °C is the largest, corresponding to its least leakage current density. In short, LNO-10 samples annealed at 600 °C exhibits superior performance, such as higher k value (>20) and the smallest leakage current 10
density (2.06 ×10-4 A/cm2). Such excellent performance will enable LNO films to be used as potential high k gate dielectrics.
Table 2. Electrical parameters of LNO-10 samples annealed at 0, 400, 500, 600 and 700
Samples
As-dep 400 500 600 700
Cox (nF) 14.0608 28.4443 36.1145 34.1743 30.0679
k
8.60 17.39 22.08 20.90 18.39
Cfb (nF)
Vfb (V)
Qox
1.93 2.07 2.10 2.10 2.08
-1.47 -0.29 0.005 0.18 0.28
6.35×10-12 6.17×10-12 5.71×10-12 4.21×10-12 3.11×10-12
(cm-2）
EOT (nm)
Jg (A/cm2) (@Vfb-1V)
7.71 3.81 3.00 3.17 3.61
4.75×10-1 4.48×10-2 1.15×10-2 2.06 ×10-4 2.22×10-3
Fig. 10. (a) C-V, (b) I-V and (d) Nyquist plots and the equivalent circuit of Pt/ LNO-10/Si/Pt MOS capacitors. (c) k and leakage current density (Jg) of LNO films as a function of annealing temperature.
4. Conclusion A required gate dielectric film needs to have both large dielectric constant and small leakage current density. This work provides a new method to significantly improve the dielectric performance of La2O3 films by Ni doping. We demonstrate that ~10.04% Ni doping exhibits superior performance, such as the largest dielectric constant of 22.08 and the least Jg value of 1.15×10-2 A/cm2. These excellent behaviors can be attributed to the smallest surface roughness (0.118 nm), the largest band gap (5.7 eV) and suitable VB and CB offsets. In addition, the performance of LNO-10 sample was optimized by varying annealing temperature. 600 °C is the 11
most suitable annealing temperature for LNO-10 films, resulting in the leakage current density as low as 2.06 ×10-4 A/cm2. This result will shed light on the design of rare earth oxide based high k materials for next-generation gate dielectrics. Acknowledgements The authors acknowledge National Key R&D Program of China (No. 2017YFB0405902) and NSFC (No. 51771002, 51971004, 21771006 and U1607126). References [1] I.A.Y. Mark T. Bohr, CMOS scaling trends and beyond, IEEE Micro 37 (2017) 21-29. [2] J. Robertson, R.M. Wallace, High-K materials and metal gates for CMOS applications, Mat. Sci. Eng. R 88 (2015) 1-41. [3] G.D. Robert M. Wallace and Wilk, High-k dielectrics and MOSFET characteristics Crit. Rev. Solid State Mater. Sci. 28 (2003) 231-285. [4] J.A. del Alamo, Nanometre-scale electronics with III-V compound semiconductors, Nature 479 (2011) 317-323. [5] J. Robertson, High dielectric constant oxides, Eur. Phys. J-Appl. Phys. 28 (2004) 265-291. [6] A.I. Kingon, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature 406 (2000) 1032-1038. [7] M.T. Bohr, R.S. Chau, T. Ghani, K. Mistry, The high-k solution, IEEES 44 (2007) 29 - 35. [8] Y.B. Yoo, J.H. Park, K.H. Lee, H.W. Lee, K.M. Song, S.J. Lee, H.K. Baik, Solution-processed high-k HfO2 gate dielectric processed under softening temperature of polymer substrates, J. Mater. Chem. C 1 (2013) 1651-1658. [9] K. Yamamoto, W. Deweerd, M. Aoulaiche, M. Houssa, S. De Gendt, S. Horii, M. Asai, A. Sano, S. Hayashi, M. Niwa, Electrical and physical characterization of remote plasma oxidized HfO2 gate dielectrics, IEEE Trans. Electron Devices 53 (2006) 1153-1160. [10] G. He, X. Chen, Z. Sun, Interface engineering and chemistry of Hf-based high-k dielectrics on III– V substrates, Surf. Sci. Rep. 68 (2013) 68-107. [11] J.W. Zhang, G. He, L. Zhou, H.S. Chen, X.S. Chen, X.F. Chen, B. Deng, J.G. Lv, Z.Q. Sun, Microstructure
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15
Ni was first incorporated into La2O3 for a novel gate dielectric. La2O3 films with different Ni doping amount are prepared by reactive sputtering. Proper amount Ni doping can improve the dielectric properties of La2O3. 10.04% Ni-doped La2O3 films show best dielectric performance. 600 °C is optimal annealing temperature for Ni-doped La2O3 films.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: