Band alignment of atomic layer deposited

Band alignment of atomic layer deposited (HfZrO4) 1−x (SiO2) x gate dielectrics on Si (100) Sung Heo, Dahlang Tahir, Jae Gwan Chung, Jae Cheol Lee, KiHong Kim, Junho Lee, Hyung-Ik Lee, Gyeong Su Park, Suhk Kun Oh, Hee Jae Kang, Pyungho Choi, and Byoung-Deog Choi Citation: Applied Physics Letters 107, 182101 (2015); doi: 10.1063/1.4934567 View online: http://dx.doi.org/10.1063/1.4934567 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/107/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Band alignment of atomic layer deposited ( ZrO 2 ) x ( SiO 2 ) 1 − x gate dielectrics on Si (100) Appl. Phys. Lett. 94, 212902 (2009); 10.1063/1.3143223 Energy-band parameters of atomic layer deposited Al 2 O 3 and HfO 2 on In x Ga 1 − x As Appl. Phys. Lett. 94, 052106 (2009); 10.1063/1.3078399 Band gap and band offsets for ultrathin ( Hf O 2 ) x ( Si O 2 ) 1 − x dielectric films on Si (100) Appl. Phys. Lett. 89, 122901 (2006); 10.1063/1.2355453 Effect of nitrogen on band alignment in HfSiON gate dielectrics Appl. Phys. Lett. 87, 212905 (2005); 10.1063/1.2135390 Relationships among equivalent oxide thickness, nanochemistry, and nanostructure in atomic layer chemicalvapor-deposited Hf–O films on Si J. Appl. Phys. 95, 5042 (2004); 10.1063/1.1689752

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APPLIED PHYSICS LETTERS 107, 182101 (2015)

Band alignment of atomic layer deposited (HfZrO4)12x(SiO2)x gate dielectrics on Si (100) Sung Heo,1,2 Dahlang Tahir,3 Jae Gwan Chung,1 Jae Cheol Lee,1 KiHong Kim,1 Junho Lee,1 Hyung-Ik Lee,1 Gyeong Su Park,1 Suhk Kun Oh,4 Hee Jae Kang,4,a) Pyungho Choi,2 and Byoung-Deog Choi2,b)

1 Analytical Engineering Group, Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, South Korea 2 College of Information and Communication Engineering, Sungkyunkwan University, Cheoncheon-dong 300, Jangan-gu, Suwon 16419, South Korea 3 Department of Physics, Hasanuddin University, Makassar 90245, Indonesia 4 Department of Physics, Chungbuk National University, Cheongju 28644, South Korea

(Received 4 June 2015; accepted 6 October 2015; published online 2 November 2015) The band alignment of atomic layer deposited (HfZrO4)1x(SiO2)x (x ¼ 0, 0.10, 0.15, and 0.20) gate dielectric thin films grown on Si (100) was obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence band offset, and conduction band offset values for HfZrO4 silicate increased from 5.4 eV to 5.8 eV, from 2.5 eV to 2.75 eV, and from 1.78 eV to 1.93 eV, respectively, as the mole fraction (x) of SiO2 increased from 0.1 to 0.2. This increase in the conduction band and valence band offsets, as a function of increasing SiO2 mole fraction, decreased the gate leakage current density. As a result, HfZrO4 silicate thin films were found to be better for advanced gate stack applications because they had adequate band gaps to ensure sufficient conduction band offsets and valence band offsets to Si. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4934567] V The rapid shrinkage of transistor feature sizes in complementary metal-oxide semiconductor (CMOS) technologies has forced the thickness of gate oxides to be reduced below the sub-nanometer scale. As a result, SiO2 and/or SiON gate oxide materials are running out of atoms; this has hampered a further scale down. Recently, the gate oxide thin films with a high dielectric constant (k) have been applied in CMOS technologies.1–7 HfO2- and ZrO2-based high-k dielectric materials have turned out to be useful materials as alternative gate dielectrics for the replacement of SiO2 in advanced semiconductor device applications. These materials are advantageous due to their composition-tunable structure, relatively high dielectric constants, and good electrical properties.3,4 HfO2 and ZrO2 have very similar physical and chemical properties to one another,5 and the correlation between the dielectric properties and lattice structures of HfO2 and ZrO2 has been studied previously.6 Especially, HfO2 has been considered to be a promising candidate to replace SiO2. However, MOS fieldeffect transistors (MOSFETs) with HfO2 as the gate dielectric suffer from threshold voltage instability, lower carrier mobility, and degraded reliability as compared to their SiO2based counterparts.8 And HfO2 typically crystallizes to produce a lower dielectric constant (k ¼ 20) in the monoclinic phase. Alternatively, ZrO2 tends to crystallize in the tetragonal phase, which is believed to have a higher dielectric constant (k ¼ 30–40).9 Triyoso et al. attempted to incorporate ZrO2 into HfO2 in order to overcome the shortcomings of HfO2. They reported that the mixing structure of HfO2 and ZrO2 (i.e., HfZrO4) is tetragonal and that it has a higher a)

[email protected] [email protected]

b)

0003-6951/2015/107(18)/182101/4/$30.00

dielectric constant than that of HfO2.10 Compared to pure compounds (e.g., ZrO2 and HfO2), the HfZrO4 films are attractive due to their higher dielectric constant, lower number of charge traps, more uniform film quality, higher capacitance, superior nMOS mobility characteristics, and good thermal stability with Si.5,8,10–12 However, compared to the HfO2 films, the gate leakage current is higher due to the smaller bandgap and lower conduction band offset in the HfZrO4 films.10,12,13 The leakage current can be controlled by combining HfZrO4 and SiO2. Fischer and Kersch showed that doping ZrO2 and HfO2 dielectric materials with Si is the most efficient stabilization method.6 Also, it has been documented that SiO2 has very stable amorphous phases and good electrical properties. When mixed with a high dielectric constant material, SiO2 decreases the number of oxygen vacancy defects and improves the electrical properties.14 For these reasons, HfZrO4 silicate (a combination of HfZrO4 and SiO2, which is referred to as (HfZrO4)1x(SiO2)x) gate dielectrics were grown by atomic layer deposition (ALD). We also prepared the HfZrO4 thin films for comparison. There are two main parameters that must be considered when attempting to reduce the gate leakage current. The first parameter is the physical thickness of the dielectrics, and the second parameter is obtaining the proper band offset with respect to Si. Therefore, the band alignment is one of the most fundamental physical properties that is used to characterize gate dielectrics.15 In order to obtain insight into the electrical properties of thin, high-k gate stacks on Si, a better understanding about the chemical and electronic band structure of high-k dielectrics is necessary. We need to identify the band gap and band offset of the dielectrics with nanometer-scale, in-depth resolution. The energy band alignment of HfZrO4 silicates and Si is fundamentally important for obtaining properly functioning

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FIG. 1. TEM images of (HfZrO4)1x(SiO2)x with x ¼ 0.10 (a) and x ¼ 0.20 (b) dielectric thin film, respectively.

transistors made out of HfZrO4 compound dielectrics. To date, the band gap and band offset values of HfZrO4 silicates have yet to be adequately investigated. Therefore, to comprehensively characterize the ability of HfZrO4 silicate dielectrics to reduce gate leakage currents, we must obtain clear information about the band offsets between the HfZrO4 silicate thin film and Si. This information will define the required barrier heights for electrons and holes. Hafnium zirconate silicate gate dielectric thin films, represented by a stoichiometry of (HfZrO4)1x(SiO2)x, were deposited by ALD. Prior to thin film deposition, a p-Si(100) substrate was cleaned chemically using the Radio Corporation of America (RCA) method.16 Zr[N(CH3) (CH2CH3)]4, Hf[N(CH3)(CH2CH3)]4, and SiH[N(CH3)2]3 were used as precursors for ZrO2, HfO2, and SiO2, respectively. O3 vapor served as the oxygen source, and N2 was supplied as the purge and carrier gas. The growing temperature was below 300 C. The SiO2-incorporated Hf-Zr-Si-O was grown by the alternate ALD process, and the number of cycles for HfO2, ZrO2, and SiO2 was repeated to mix the SiO2 content in a controlled manner. In order to examine the relationship between the composition and the bandgap energy and band offsets, the SiO2 mole fraction was varied (x ¼ 0.10, 0.15, and 0.20). The physical thickness of the deposited layer was 6.5 nm 6 0.3 nm, which shows the uniformity of the thickness for different mole fractions (x) of SiO2 from 0.1 to 0.2 of silicate thin films, and it clearly show the abruptness of the interface as shown in the TEM images in Fig. 1. We used reflection electron energy loss spectroscopy (REELS) to investigate the electronic structures near the band gap of the HfZrO4 silicate films with a variety of

compositions. REELS is a good tool for investigating the electronic structure (e.g., the band gap) of ultrathin dielectric films because the low energy loss region spectrum reflects the structure of the valence and conduction bands of the samples. By combining the REELS spectra with the results from the X-ray photoelectron spectroscopy (XPS) analysis, we can explain the composition dependence of the band alignment in the HfZrO4 silicate dielectric layer. XPS and REELS spectra were obtained with a VG ESCALAB 210. XPS spectra were measured using a Mg source with a pass energy of 20 eV. The incident and takeoff angles of electrons were 55 and 0 from the surface normal, respectively. The binding energies were referenced to the C1s peak of the hydrocarbon contamination at 285 eV. The REELS spectra were measured with a primary electron energy of 1.5 keV for excitation and with a constant analyzer pass energy of 20 eV. The full width at half maximum (FWHM) of the elastic peak was 0.8 eV. Figure 2 shows the (a) Zr3d, (b) Hf4f, and (c) Si2p photoelectron core level spectra for HfZrO4 and the HfZrO4 silicate thin films. When the HfZrO4 content was higher, the peak intensity was higher. Figure 1(a) shows the binding energies of the Zr3d5/2 and Zr3d3/2 peaks in the HfZrO4 thin film, which were measured at 181.7 and 184.1 eV, respectively. These two peaks had a spin-orbital splitting of 2.4 eV.10 For (HfZrO4)1x(SiO2)x thin films, the binding energy of Zr3d was shifted to a higher binding energy as the Si content in the silicate was increased. All of the Zr3d core levels showed a clear peak shift compared to pure HfZrO4. From Fig. 2(b), the peak positions of Hf 4f7/2 were 17.6, 17.9, 18.1, and 18.3 eV; these peaks can be matched to HfZrO4, (HfZrO4)0.9(SiO2)0.1, (HfZrO4)0.85(SiO2)0.15, and

FIG. 2. (a) Zr3d, (b) Hf4f, and (c) Si2p core level photoelectron spectra for (HfZrO4)1x(SiO2)x (x ¼ 0, 0.10, 0.15, and 0.20) dielectric thin films.


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FIG. 3. (a) Reflection electron energy loss spectra with primary energies of 1500 eV and (b) valence band spectra for (HfZrO4)1x(SiO2)x (x ¼ 0, 0.10, 0.15, and 0.20) dielectric thin films.

(HfZrO4)0.8(SiO2)0.2, respectively. These results indicated the existence of the silicate structure. It was observed that the Hf4f and Zr3d peaks became slightly broader upon the incorporation of more SiO2. Additionally, the HfZrO4 silicate with x ¼ 0.1 (i.e., a higher HfZrO4 fraction) showed sharp Hf4f and Zr3d peaks. For silicates, the core level peaks shifted to a higher binding energy due to charge transfer. According to Pauling’s theory, the charge transfer out of Hf and Zr in HfZrO4 silicate is larger than that in HfZrO4 because the electronegativities of both Si and O are larger than those of Zr and Hf. From the shift of the XPS peak energy, we can argue that the binding energy position of the Hf4f and Zr3d core levels are sensitively dependent on charge transfer to surrounding O atoms. The shift to a higher binding energy of the Zr3d peak was suggestive of Zr silicate formation. Figure 2(c) shows the Si2p core level spectra for the HfZrO4 silicate dielectrics. For (HfZrO4)0.8(SiO2)0.2, the Si2p peak (located at 102.1 eV) was found to be reduced by 1.2 eV when compared to that of SiO2 (at 103.3 eV).17 The Si2p spectra shifted to a lower binding energy as the amount of HfZrO4 in the silicate increased. The binding energy shift of these core level spectra is consistent with the electronegativity equalization model based on the relative atomic electronegativities of Hf (1.30), Zr (1.33), O (3.44), and Si (1.90). By investigating the chemical shifts of the Hf4f, Zr3d, and Si2p core level spectra using XPS, the formation of HfZrO4 silicates was confirmed. We utilized REELS measurements to determine the band gaps. Figure 3(a) shows the REELS spectra for HfZrO4 and HfZrO4 silicate dielectric films. The onset of loss was due to electron-hole excitation and corresponded to the band gap value of the dielectric film. The band gap energy was thus found by drawing a linear fit line with the maximum negative slope from a point near the onset of the loss spectrum to the background level. The intersection point provided the band gap value. This method has been described previously.18–20 The band gap was determined from the average of several measurements and the uncertainty, as

calculated from the standard deviation, was 60.1 eV. The band gap values for these high-k gate dielectric films are shown in Table I. We have also added the band gap values of ZrO2, HfO2, and SiO2 (as found in a previously published paper) for comparison.18,19 It is interesting to note that the band gap of the HfZrO4 thin film was determined to be 5.40 eV,20 which is in between the values of the ZrO2 thin film (5.30 eV)18 and the HfO2 thin film (5.50 eV).19 Despite the addition of 10% SiO2 (mole fraction) in the HfZrO4 thin film, the band gap values of HfZrO4 silicate barely changed; however, for the additions of 15 and 20 at % SiO2 into HfZrO4, the band gap increased to 5.60 eV and 5.80 eV, respectively. For the HfZrO4 gate oxide, plasmon peaks appeared at 15, 26, 35, 42, and 47 eV, as indicated in Fig. 3(a). The strongest plasmon peak appeared at 15 eV and broad plasmon peaks appeared near 26 eV and 47 eV, away from the elastic peak center.18,19 For (HfZrO4)1x (SiO2)x thin films, the intensities of the plasmon peak at 15 eV decreased with increasing SiO2 content. This implies that the plasmon peak at 15 eV could be attributed to HfZrO4, while the plasmon peak near 23 eV mainly originated from the presence of SiO2.18,19 Increasing the mole fraction of SiO2 caused the 2nd plasmon peak to move to a lower loss energy. The band gap width is defined as the difference between the bottom of the conduction band and the top of the valence band. It is well known that the conduction band minimum state is formed from the Si2s state and that the valence band maximum (VBM) states are formed from the O2p states.21 For transition metal oxides, the unoccupied orbital is related to the localized d states. Compared to SiO2, the band gap of HfZrO4 silicate is reduced. The obtained band gap values suggested that the localized d electron state (introduced by Hf and Zr) was located below the conduction band minimum of SiO2. It is interesting to note that the localized d electron state of HfZrO4 is in between those of the Hf and Zr d electron states. In SiO2incorporated HfZrO4 silicates, the band gap increased with increasing SiO2. The band gap for the HfZrO4 silicate thin films was mainly determined by the hybrid Hf 5 d-Zr 4 d conduction band state and the O 2 p valence band.

TABLE I. Band gap and band offset values for gate oxide films. Energy (eV) Eg DEv DEc

ZrO2 (Ref. 18)

HfZrO4 (Ref. 20)

HfO2 (Ref. 19)

(HfZrO4)0.90 (SiO2)0.10

(HfZrO4)0.85 (SiO2)0.15

(HfZrO4)0.80 (SiO2)0.20

SiO2

5.30 2.35 1.83

5.40 2.50 1.78

5.50 2.86 1.54

5.40 2.50 1.78

5.60 2.60 1.88

5.80 2.75 1.93

9.00 4.28 3.60


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To obtain the valence band offset at the dielectric/Si interface, the valence band spectra were measured in order to determine the VBM. These spectra are shown in Fig. 3(b). The VBM is determined from the intersection of two straight lines, where one line fits the valence band leading edge and the other line fits the background. For (HfZrO4)1x(SiO2)x thin films, the spectral intensity was spread over a range of binding energies (5–8 eV) upon the introduction of SiO2. The spectral intensity also shifted to a higher binding energy. From the energy difference between the VBM of the gate dielectric and that of Si, the valence band offset was obtained. The valence band maximum of Si was positioned at 0.24 eV. The valence band offset (DEv) for HfZrO4 was 2.50 eV, which was higher than that of ZrO2 (2.35 eV)18 and lower than that of HfO2 (2.86 eV).19 The valence band offsets increased upon the introduction of SiO2 in HfZrO4 and became even larger as more SiO2 was added. The valence band offsets were determined to be 2.50, 2.60, and 2.75 eV for (HfZrO4)0.9(SiO2)0.1, (HfZrO4)0.85(SiO2)0.15, and (HfZrO4)0.8(SiO2)0.2, respectively. The band gap (Eg) in Fig. 3(a) and the DEv value in Fig. 3(b) allowed us to determine the conduction band offset (DEc) by using the following relationship:18,19 DEc ¼ Eg ððHfZrO4 Þ1x ðSiO2 Þx Þ DEv ððHfZrO4 Þ1x ðSiO2 Þx =SiÞ Eg ðSiÞ: The conduction band offset for HfZrO4 was 1.78 eV, which was lower than that of ZrO2 (1.83 eV)18 and higher than that of HfO2 (1.54 eV).19 The conduction band offsets were determined to be 1.78, 1.88, and 1.93 eV for (HfZrO4)0.9 (SiO2)0.1, (HfZrO4)0.85(SiO2)0.15, and (HfZrO4)0.8(SiO2)0.2, respectively. The band alignment parameters for these dielectrics are listed in Table I. It is critically important that a high-k gate dielectric has a conduction band offset and a valence band offset of at least 1 eV (relative to silicon) to allow for both electrons and holes to inhibit conduction via Schottky emissions of carriers into their bands.8 The DEc values for these HfZrO4 silicate films satisfy the minimum requirements for the barrier heights (i.e., greater than 1 eV) in devices. Additionally, we note that the conduction band offsets for HfZrO4 silicate thin films depend on the SiO2 content. These changes in the conduction band offsets affected the electrical properties of the gate leakage current. Figure 4 shows the leakage current density of HfZrO4 silicates as a function of the applied gate voltage. The leakage current density decreased as the SiO2 content was increased. At a gate voltage of 1 V, the HfZrO4 silicate thin film with a content ratio of x ¼ 0.2 showed the lowest leakage current density (4.88 106 A/ cm2) while a content ratio of x ¼ 0.1 showed the highest leakage current density (5.06 104 A/cm2). The increasing valence band offset and conduction band offsets (with increasing SiO2) decreased the gate leakage current density. In summary, we investigated the band alignment for HfZrO4 silicates via REELS and XPS analyses. Our results showed that, for (HfZrO4)0.9(SiO2)0.1 and (HfZrO4)0.8(SiO2)0.2, the band gap slightly increased from 5.4 eV to 5.8 eV and the valence band offset also increased from 2.50 eV to 2.75 eV. Therefore, as the SiO2 content in the HfZrO4 silicates was

Appl. Phys. Lett. 107, 182101 (2015)

FIG. 4. Gate leakage current density of (HfZrO4)1x(SiO2)x thin films with changes in the SiO2 mole fraction (x).

varied, the conduction band offset was increased from 1.78 eV to 1.93 eV. Thus, the band gap values can be controlled by selecting the appropriate Hf and Zr metal compositions in order to obtain an optimum value. As the SiO2 content increased, the gate leakage current density decreased; this is caused by the increased conduction band offset and valence band offset, which caused a reduction in the electron and hole emission from the Si substrate. In this study, we found that HfZrO4 silicates were much better for advanced gate stack applications because they had adequate band gaps that ensured sufficient conduction band offsets and valence band offsets to Si, which yielded a substantially reduced gate leakage current density. 1

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