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Ge gate stacks by nitrogen incorporation
Journal of Alloys and Compounds 695 (2017) 2199e2206
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interface quality modulation, band alignment modification and optimization of electrical properties of HfGdO/Ge gate stacks by nitrogen incorporation J. Gao a, b, G. He a, *, Z.B. Fang c, **, J.G. Lv d, ***, M. Liu e, Z.Q. Sun a a
School of Physics and Materials Science, Radiation Detection Materials & Devices Lab, Anhui University, Hefei 230039, China School of Sciences, Anhui University of Science and Technology, Huainan 232001, China Department of Physics, Shaoxing University, Shaoxing 312000, China d Department of Physics and Electronic Engineering, Hefei Normal University, Hefei 230061, China e Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2016 Received in revised form 23 October 2016 Accepted 5 November 2016 Available online 8 November 2016
Effects of nitrogen incorporation on the interfacial chemical bonding states, band alignment, electrical properties and leakage current conduction mechanisms of HfGdO-based metal-oxide-semiconductor (MOS) capacitors has been investigated by UVeVis transmission spectroscopy, X-ray photoemission spectroscopy (XPS), and electrical measurements. XPS results indicate that incorporation of a moderate amount of nitrogen incorporation into HfGdO gate dielectrics can effectively suppress the formation of low-k GeO2 and Hf(Gd)-Ge-O interfacial layer at the interfacial region. Meanwhile, reduction in band gap and valence band offset and increase in conduction band offset have been observed after nitrogen incorporation. Electrical measurements based on MOS capacitors have shown that MOS capacitor with HfGdON/Ge stacked gate dielectric with N2 flow rate of 3 sccm exhibits small gate leakage current (1.08 103 A/cm2 at Vg ¼ 1 V), almost disappeared hysteresis, and large dielectric constant (29.2). The involved leakage current conduction mechanisms for MOS capacitor devices with and without nitrogen incorporation also have been discussed in detail. © 2016 Elsevier B.V. All rights reserved.
Keywords: High-k gate dielectrics Interface quality Band alignment Electrical properties Leakage current mechanism
1. Introduction In order to meet the requirements of high efficiency and low energy consumption of electrical devices, Si-based metal-oxidesemiconductor field-effect-transistor (MOSFET) have been scale down to their physical limit. To further improve the device performance, germanium (Ge) has been studied as a leading candidate to replace silicon (Si) due to its higher carrier mobility and narrower band gap, as compared to Si [1]. However, the absence of high quality MOS interface compared to that of SiO2/Si interface retards the development of Ge-based MOSFETs devices with high performance. Fortunately, recent progress made in the direct
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. He), [email protected] (Z.B. Fang), [email protected] (J.G. Lv). http://dx.doi.org/10.1016/j.jallcom.2016.11.068 0925-8388/© 2016 Elsevier B.V. All rights reserved.
deposition of high-k gate dielectrics due to its superior thermodynamic stability when in contact with Ge substrate have reopened the possibility of realizing Ge-based CMOS devices, overcoming its poor interface quality issues. In recent years, HfO2 as gate dielectrics on Ge has been paid more attention to complementary mobility degradation [2,3]. However, their moderate dielectric constants make Hf-based gate dielectrics limited for future CMOS device scaling with an EOT below 1.0 nm [4]. One of the effective ways to increase Hf-based dielectric permittivity is combining it with another much higher dielectric materials. It has been reported that doping of rare earth oxide (REO) into HfO2, such as La2O3, Pr2O3, and Gd2O3, can improve its physical and electrical properties [5e8]. Especially, more attention has been paid to investigate the Gd2O3 incorporated HfO2 high-k gate dielectrics. Gd2O3 has some advantages, such as relatively high dielectric constant, excellent thermal stability, and almost no hysteresis in MOS devices [9]. Additionally, the conduction band offset (CBO) Of Gd2O3 is greater than that of HfO2. Thus, doping Gd2O3 into HfO2 will increase the
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CBO and improve band offset symmetry [10,11]. Meanwhile, doping nitrogen into high-k gate dielectric oxides can suppress oxygen diffusion through high-k oxide effectively, inhibit interfacial reaction, increase crystallization temperature and passivate oxygen vacancy states [12,13]. But incorporation of nitrogen also brings some disadvantages, such as reduction of band gaps, larger leakage current [14]. Xiong et al. have reported the effect of Gd and N incorporation on the band structure and oxygen vacancies of HfO2 gate dielectric films deposited on Si substrates, and confirmed that the oxygen vacancies are greatly suppressed by Gd and N co-doping under the suitable doping amount [15]. Based on previous publication and observations, it can be noted that the co-doping of Gd and N into HfO2may help to extend the application of Hf-based oxides beyond 16 nm and 11 nm technology nodes due to their symmetric band offsets and observably reduced defect states in the band gap [14]. In spite of these improved interfacial and electrical properties of Gd/N-codoped HfO2 gate dielectrics, these investigations are still focused on the Si-based MOS device. Current, quick development in microelectronics area has pushed the application of Ge-based device integrated with Hf-based high-k gate dielectrics. By far, much less work has been devoted to the investigation of Ge-based MOS devices based on Gd/N-codoped HfO2 high-k gate dielectrics. Systematical investigation and observation of interfacial and electrical characteristics of HfO2/Ge gate stack modulated by Gd/N-codoping is necessary. Additionally, the nitrogen incorporation effect of on the band alignments, electrical properties and leakage current conduction mechanisms of HfGdO(N)/Ge gate stack in particular, has not been systematically investigated. As we know, as an important parameter to determine the electrical performance of FETs, the determination of the band alignment of high-k/Ge hetero structures is needed for a complete FETs technology. In current work, more attention has been paid to the investigation of the interfacial chemical bonding states and electrical characteristics of HfGdO(N)/Ge gate stack as a function of nitrogen incorporation content. Additionally, the evolution of the band offset, as well as the leakage current conduction mechanisms of HfGdO and their nitrided derivatives has been discussed in detail. 2. Experimental Sb-doped n-type Ge (100) wafers were cleaned in acetone for 5 min at 75 C to rewove organic contamination followed by ethanol solution, then washed with a mixed solution (NH3$H2O:H2O ¼ 1:4) for 5 min at room temperature, and cleaned in H2O2 solution (H2O2:H2O ¼ 1:5) for 60 s to form oxides layer, which is followed NH3$H2O solution for 5 min to remove the oxides layer, then blown dry with pure N2. After cleaning, ex-situ HfGdO and HfGdON gate dielectric films were deposited by sputtering equipment (JGP-DZS, Chinese Academy of Sciences, Shenyang Scientific Instrument Co., Ltd) using HfGdO ceramic target (atomic ratio of Hf:Gd ¼ 9:1) in a mixed ambient of Ar and N2. The different N incorporated HfGdON films were obtained by changing the flow rate of Ar/N2 mixture to 20:0, 20:3, 20:7, and 20:10 (sccm), which were assigned as S1, S2, S3, and S4, respectively. The base vacuum was 5.0 104 Pa. The RF power and the working pressure were kept at 60 W and 0.5 Pa during the sputtering process. Physical thickness of the gate dielectric layer was measured by spectroscopic ellipsometry (SC630, SANCO Co, Shanghai). About 40-nmthick HfGdO and HfGdON films were deposited on quartz substrate to obtain their optical band gaps (Eg) by transmittance using ultravioletevisible spectrophotometer (UVevis) techniques. The thickness of HfGdO and HfGdON films for X-ray photoelectron spectroscopy (XPS) measurement was about 4 nm. Ex-situ XPS measurements for HfGdON films were performed by using (ESCALAB 250Xi) system, equipped with an Al Ka radiation source
(1486.6 eV) and hemispheric analyzer with a pass energy of 20 eV. All the collected data were corrected using the binding energy of C 1s peak (284.8 eV). Spectral deconvolution was performed by Shirley background subtraction using a Voigt function convoluting the Gaussian and Lorentzian functions. In order to explore the electrical properties, Al/HfGdO(N)/Ge/Al MOS capacitors were fabricated by sputtering a Al top electrode through a shadow mask with an area of 3.14 104 cm2 and the back surfaces of all samples were deposited with a 200-nm-thick Al by sputtering after the back surface oxide stripping to decrease contact resistance. A semiconductor device analyzer (Agilent B1500A) combined with Cascade Probe Station was used for CeV measurement at room temperature. Short circuit and open circuit calibration were performed before real measurements. Additionally, the leakage current properties were measured by B1500A. All the electrical characterization was performed at room temperature in a shielded dark box. 3. Results and discussion 3.1. Interfacial bonding states analysis To investigate the effect of nitrogen incorporation on the chemical states of HfGdO gate dielectrics, Hf 4f, Ge 3d, N 1s and O 1s core-level spectra have been analyzed by XPS, as shown in Fig. 1. Among of them, Fig. 1(a) shows the core level survey spectra of HfGdO(N)/Ge gate stack at a pass energy of 150 eV and a take-off angle of 45 . It can be seen that all characteristic peaks are derived from Hf, Gd, N, O and Ge elements as well as a small amount of C component introduced from thin film growth or air contamination during the measurement, indicating that the asobtained films were HfGdO and HfGdON films. To confirm the nitrogen incorporation into HfGdO gate dielectrics, N 1s core-level XPS spectra of as-deposited HfGdON films with different nitrogen incorporation have been investigated, as demonstrated in the inset in Fig. 1(a). Based on Fig. 1(a), it can be noted that nitrogen incorporated into the HfGdO films has been observed. In the case of the Ge 3d spectra shown in Fig. 1(b), some distinctive sub peaks are identified. The two broad peaks correspond to Ge0 component and a mixture of multiple Ge oxides chemical bonding states, which can be decomposed into four contributions: three are sub-oxides of Ge, i.e., Ge2O (Ge1þ), GeO (Ge2þ), Ge2O3 (Ge3þ), and the one is native oxide GeO2 (Ge4þ). And there is an additional bonding state component, Ge*, i.e., germanate (Hf-Ge-O), between the Ge3þ and Ge4þ components [16]. Compared with S1 sample, it is clearly seen that for sample S2, the peak corresponding to Ge oxides shifts to lower binding energy. Meanwhile, the relative proportion of the Ge oxides increase. The change may be due to the formation of a large amount Ge suboxide with lower binding energies and reduction of germanate with higher binding energy at the interface between HfGdON film and Ge substrate with the incorporation of a small amount of nitrogen. Additionally, the content of native oxide GeO2 for sample S2 almost keeps unchanged compared with sample S1. With the flow rate of N2 increasing from 7 to 10 sccm, the Ge oxides peak shifts to higher binding energy, which can result from the reduction of Ge suboxide and increase of germanate and native oxide GeO2 with higher binding energies. Therefore, it can be concluded that germanate and native oxide can be effectively suppressed after moderate N doping into HfGdO with N2 flow rate of 3. To further identify the effect of nitrogen incorporation on the interfacial chemical bonding state of HfGdO/Ge gate stack more clearly, O 1s core-level spectra have been investigated, as shown in Fig. 1(c). All spectra are deconvoluted into three peaks located at binding energy of 530 eV, 531.1 eV, and 532.1 eV, corresponding to
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Fig. 1. XPS analysis of HfGdO and HfGdON thin films with different nitrogen incorporation concentration (a) Core level XPS survey spectra (Inset is the corresponding N 1s XPS core level spectra.); (b) Ge 3d XPS core level spectra; (c) O 1s XPS core level spectra; (d) Hf 4f XPS core level spectra.
Hf(Gd)-O, Ge-O, and germanate (Ge-Hf(Gd)-O), respectively [17]. From Fig. 1(c), it can be seen that intensity of Ge-Hf(Gd)-O decreases significantly after N doping into HfGdO with N2 flow rate of 3. The change may be due to the incorporation of nitrogen into HfGdO gate dielectrics and suppress the oxygen diffusion at the HfGdON/Ge interface. Meanwhile, the intensity of Ge-O increase, which can be attributed to the formation of a large amount Ge suboxide. However, with the flow rate of N2 increasing from 7 to 10 sccm, increase in the Ge-Hf(Gd)-O content and reduction in the Ge-O content have been detected. So it is obvious to see that germanate interfacial layer could not be suppressed effectively if too much nitrogen is incorporated into high-k gate dielectrics. In order to further confirm the evolution of the interfacial chemical bonding states related with nitrogen incorporation, Hf 4f core-level spectra have been paid attention, as demonstrated in Fig. 1(d). All of Hf 4f spectra are decomposed into two peaks of Hf 4f5/2 and Hf 4f7/2, which can be fitted well by spin orbit splitting energy of about 1.7 eV [18]. The Hf 4f7/2 peak of the S1 sample without doping N lies at 18.0 eV, and those of the S2eS4 samples shift to lower binding energy, which may be due to the reduction of Hf germanate at the interface between HfGdON film and Ge substrate with the incorporation of nitrogen. However, it is worth noting that with the flow rate of N2 increasing from 3 to 10 sccm, the peaks of Hf 4f shift to higher binding energy sides, which may be due to the fact that the formation of Hf germanate can't be inhibited by further increasing the N2 flow rate [19]. Based on above analysis, it can be concluded that appropriate amount of N doped into HfGdO can suppress the formation of germanate and native oxide GeO2. However, too much N doped into HfGdO can promote the generation of germanate and native oxide GeO2. So it is important to control the flow rate of N2 to improve the interfacial properties between HfGdON films and Ge substrates.
3.2. Determination of band gap and band alignment investigation The magnitude of the band offset (BO) between the high-k oxide film and semiconductor is the most fundamental properties for the reliability of MOS devices. In order to obtain the band offset of the interface between HfGdO(N) films with the Ge substrate, the valence-band offset spectra were investigated. To determine the valence band offsets of samples with different flow rate of Ar:N2, the optical band gap (Eg) of all samples were extracted from the UVeVis transmission spectroscopy. Based on the Tauc relation [20], the optical energy band gap (Eg) values of HfGdO(N)/Ge gate stacks are determined by the following equation:
ahn ¼ A hn Eg
1=2
;
(1)
in which A is a constant characteristic of the semiconductor, a, h, and n is absorption coefficient, Planck constant, and frequency of light, respectively. As shown in Fig. 2, the Eg can be obtained by extrapolating the linear part of the (ahn)2 versus hn curves to the energy axis. With the increase of N doping content, the Eg of HfGdON films gradually shifts to lower energy, ranging from 5.85 to 5.42 eV with the N2 flow rate changing from 0 to 10 sccm, respectively. As we know, the band gap is determined by the difference between the valence band maximum and conduction band minimum. The decrease in Eg value could be attributed to the increase of N doping composition in the HfGdON films. For HfGdON films, the valence band maximum is mainly formed by the mixing of N 2p and O 2p states. Since N 2p state locates above the O 2p state, the increase of N doping content leads to the increase of the valence band maximum of HfGdON thin films [15]. Kraut's method has been used to determined the valence-band
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DEC ðHfGdON GeÞ ¼ Eg ðHFGdONÞ DEV ðHfGdON GeÞ Eg ðGeÞ
Fig. 2. Determination of band gaps of HfGdO and HfGdON thin films with different nitrogen concentration.
offset at the HfGdON/Ge interface [21]. According to the Kraut's method, the valence band alignment of HfGdON films on Ge substrates could be determined by measuring the valence band maximum (VBM) difference between HfGdON films and the Ge substrates, as expressed in following relation:
DEV ðHfGdON GeÞ ¼ EV ðHfGdONÞ EV ðGeÞ
(3)
where DEc (HfGdON-Ge) is the conduction band offset, Eg (HfGdON) is the band gap of HfGdON film, and Eg (Ge) is the band gap of the Ge substrate taken as 0.66 eV. By combining the band gap values and DEv values, the schematic energy band alignment of HfGdON/ Ge gate stacks corresponding to different N doping corporation is summarized as shown in Fig. 4. The obtained DEc values are 2.36, 2.41, 2.32, and 2.25 eV for HfGdON films with the N content of 0, 3, 7, and 10 sccm, respectively. According to band offset analysis, it can be concluded that the valence-band offset as well as the conduction-band offset to the Ge substrates of the HfGdON are greater than 1 eV, which makes it suitable as a candidate for high-k gate dielectrics in Ge-based MOS devices. Compared with the sample S1 (without N doping), the DEc of the S2 sample increases and those of the S3 and S4 samples decrease. The increased DEc is considered as an important factor to reduce leakage current effectively for COMS devices. So it can be assumed that by doping the appropriate amount of N into HfGdO, the valence band offset could be increased, which can effectively reduce the leakage current. However, with the amount of N doping into HfGdO increasing from 7 to 10 sccm, the band gap (Eg) continue decreases. Meanwhile, the valence band offset decrease, which would lead to the uncontrollable leakage current and degraded device performance.
(2)
The Ev is determined by using the linear extrapolation method as shown in Fig. 3. The valence band energy Ev(Ge) is located at 0.28 eV. The valence band offset values (DEv) of HfGdON are 2.82, 2.64, 2.54, and 2.50 eV, with the N2 flow rate changing from 0, 3, 5 to 10 sccm, respectively. It is obviously that the values of DEv decrease with the increase of N doping composition into HfGdON. The VB of HfGdO film is mainly determined by O 2p states. After N doping into HfGdO thin films, the contribution of N 2p state, which lies above that of O 2p state, cannot be ignored. As a result, the VB of HfGdON film would raise the valence band maximum and reduce the valence band offset of the films. Moreover, the more N content in the HfGdON thin films, the smaller VB offsets [15]. The conduction band offset (DEc) can be obtained by simply subtracting the valence band offset and the energy gap of the Ge substrate from the band gap of a HfGdON film, as expressed in the following equation:
Fig. 3. The difference between the VBM of HfGdO(N) and Ge substrate with different nitrogen concentration.
3.3. Electrical properties characteristics Fig. 5 demonstrates the high-frequency (1 MHz) CeV characteristics of the Al/HfGdON/Ge MOS capacitors with different Ndoping content. It can be noted that obvious changes have taken
Fig. 4. Schematic band diagram of HfGdO and HfGdON thin films with different nitrogen concentration.
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Fig. 5. Capacitanceevoltage (CeV) characteristics of HfGdO and HfGdON films with different nitrogen doping content. The inset is the CeV curve of HfGdON film with the N flow rate of 3 sccm.
place in the CeV curves after nitrogen incorporation, indicating the change in quality of HfGdON films with increase in N content. For S1 sample, an obvious hump exists in the inversion region and a kink exists in depletion region, which can be caused by slow interface states [22]. However, the hump and kink have been observed to disappear after doping a small amount of N (flow rate of N is 3 sccm), indicating that N-doping into HfGdO prevents the formation of slow interface states. But with the N content increasing from 7 to 10 sccm, the hump once again appears in the inversion region, implying that too much nitrogen is not good for the improvement of the interface quality between HfGdON films with Ge substrates. The relative dielectric constants (k) of the four samples are extracted from the accumulation capacitance without taking into account quantum mechanical corrections, which are 27.8, 29.0, 26.9 and 26.5 with the N2 flow rate changing from 0, 3, 5 to 10 sccm, respectively. All of k values agree with those of HfGdO(N) films reported in previous works [23]. In addition, the extracted positive flat band voltage Vfb from CeV curves suggests that there are some negative native defect or trap oxide charge in the oxide layer and/or at the interface [24]. Compared with S1 sample, S2 sample shows an obvious negative Vfb shift, indicating the less negative oxide charges, which can be attributed to either singly and doubly negatively charged interstitial oxygen atoms, or reduction of defect traps in the film and near the interface [25,26]. However, with increased N flow rate, S3 and S4 sample exhibit obvious negative Vfb shift, which indicates the more positive oxide charges exist in the oxide layer and/or at the interface. The k value, Vfb of four samples are listed in Table 1. The leakage current density versus voltage (JeV) curves for Al/ HfGdO(N)/Ge MOS capacitors with voltage range of 3 V ~ 3 V were shown in Fig. 6. It can be noted that the S2 sample processes the smallest leakage density of about 1.08 103 A/cm2 at 1 V, which can be due to the smallest interface state densities and smallest oxide trapped charge at/near interface, as shown in Table 1. Moreover, another plausible explanation for smallest leakage current is the largest DEc confirmed by previous XPS analysis. Based on electrical analysis and XPS analysis, it can be summarized that
Table 1 Parameters of the Al/HfGdO(N)/Ge MOS capacitors extracted from CeV curves. Samples
EOT (nm)
k
Vfb (V)
J (A/cm2)
S1 S2 S3 S4
1.68 1.60 1.76 1.76
27.8 29.2 26.9 26.5
0.39 0.31 0.44 0.52
3.01 1.08 3.57 2.67
103 103 102 101
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Fig. 6. Leakage current density vs gate voltage (JeV) characteristics of HfGdO and HfGdON films with different N doping content.
HfGdON with the N2 flow rate of 3 sccm exhibits the best interfacial quality and best electrical properties. The largest accumulation capacitance and the smallest leakage current in S2 sample can result from the smallest oxygen-vacancy-related interface states and the biggest conduction band offset. Compared with HfO2 films directed deposited on Ge substrates reported in previous works [27e29], the improvement of capacitance voltage hysteresis and leakage current density have been observed in HfGdO(N) samples. There is almost no hysteresis voltage in HfGdO(N) samples [27,28], which is attributed to the doping of Gd [9]. The leakage current density is reduced by four orders of magnitude from the order of 101 A/cm2 to 103 A/cm2 [28]. On the basis of above studies, S2 sample (HfGdON with the N2 flow rate of 3 sccm) with the best performance has been selected to investigate the post-deposition annealing effects on electrical properties. After deposition of HfGdON films, rapid thermal annealing (RTA) process was performed at 300, 400 and 500 C for 60s in a vacuum of 6.3 105 bar, respectively. Then, Al/HfGdON/ Ge/Al MOS capacitors were fabricated by sputtering a Al top electrode with an area of 3.14 104 cm2. The high-frequency (1 MHz) CeV curves and the leakage current density versus voltage (JeV) curves of the Al/HfGdON/Ge MOS as a function of annealing temperatures are shown in Fig. 7. From Fig. 7(a), it can be seen that the CeV curves of annealed samples as well as as-deposited sample are almost no hysteresis. All annealed samples show a continually positive Vfb shift compared to the as-deposited sample with annealing temperature increasing, which indicates the generation of more negative interstitial oxide charges in the oxide layer and/or at the interface during annealing [23]. The relative dielectric constant (k) of the samples extracted from the accumulation capacitance are 29.2, 24.9, 25.6, and 26.7 for the as-deposited and annealed samples from 300 to 500 C, respectively. The values of k decreases after annealing process. The small dielectric constant may be resulted from the microstructure change such as interface property and defects in the film, etc [30,31]. Based on above analysis, it can be drawn that the interface property become worse during annealing process. This conclusion can be further confirmed in the JeV curve shown in Fig. 7(b). From JeV curves, it can be seen that the ASD sample processes the smallest leakage density of about 1.08 103 A/cm2 at 1 V, which can be due to the best interface quality and smallest oxide trapped charge at/near interface. In a words, RAT process in the vacuum does not improve the film quality; on the contrary, annealing process causes deterioration of film properties. Much more work is needed to investigate the effect of the post-annealing temperature
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Fig. 7. (a) Capacitanceevoltage (CeV) characteristics of Al/HfGdON/Ge gate stacks with the N flow rate of 3 sccm annealed at different temperature; (b) Leakage current density vs gate voltage (JeV) characteristics of Al/HfGdON/Ge gate stacks with the N flow rate of 3 sccm annealed at different temperature.
on electrical properties. 3.4. Charge conduction mechanisms in oxides Based on JeV measurements, it has been noted that in HfGdO(N)/Ge gate stacks J under gate injection (V > 0) is much larger than that under substrate injection (V < 0) at same absolute gate voltage, which is attributed to the different energy band structure under different injection mode [32]. JeV curves are different, suggesting that carrier transportation mechanisms are different in the four as-investigated samples. In order to understand the influence of N doping concentration on the carrier transportation mechanisms of HfGdO(N)/Ge gate stacks, several different current conduction mechanisms were explored for both substrate injection and gate injection in current work, including Poole-Frenkle (P-F) emission, Schottky emission (SE), and FowlerNordheim (F-N) tunneling [33]. Fig. 8(a)e(d) show the fitting lines about leakage current
densities (J) as a function of the applies field (E) of HfGdO(N) films with different N-incorporated content. Through analysis, the similar phenomenon can be found in all samples. In the case of gate injection, the main current conduction mechanisms should be P-F emission as well as SE in the region of lower electric fields, and F-N emission in the region of high electric fields, respectively. For substrate injection, the main current conduction mechanism is F-N emission in the region of higher electric fields. The detailed analysis is described in the following section. It can be determined that for gate injection, the dominant current conduction mechanism is P-F emission in the region of lower field region (jEj~1.35e1.6 MV/cm), which correlated with trap and field assistant current. The P-F emission can be expressed as: [33]
JPF
2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 q ft qE=pε0 εox 5 ¼ AE exp4 kB T
(4)
Fig. 8. Leakage current mechanism for the Al/HfGdO(N)/Ge MOS capacitor with different N doping content. (a) Poole-Frenkle (P-F) emission plots for gate injection; (b) Schottky emission (SE) plots for gate injection; (c) Fowler-Nordheim (F-N) tunneling plots for gate injection; (d) Fowler-Nordheim (F-N) tunneling plots for the substrate injection.
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in which Ft is the trap energy level below the conduction band edge of the dielectric, εox is the oxide dielectric constant [33]. Similarly, ln(J/E) should be proportional to the E1/2. Fig. 8(a) shows that the plot of ln(J/E) versus the E1/2 is good linear fitting for all samples, which indicates the P-F emission through the oxide layer. The slope of the ln(J/E)-E1/2 curve can be deduced from P-F emission expression as follows:
1 slope ¼ kT
sffiffiffiffiffiffiffiffiffiffiffiffiffiffi q3 pε0 εox
(5)
from which the dielectric constant εox of HfGdO(N) films can be obtained. The dielectric constant εox extracted from the slope of P-F fitting line under gate injection. The values are 11.1, 6.4, 24.5 and 22.6 for the S1, S2, S3 and S4 samples, respectively. Compared with previous results reported, it can be seen that the obtained εox of S1 and S2 samples derive from experimental data, those of S3 and S4 agree with the electric constant obtained from experiments. The results indicate that for S3 and S4 samples, the P-F emission is the denominated current conduction mechanisms for the gate injection in the medium electric field region. For all samples, it has been found from Fig. 8(b) that in the region of lower electric fields (jEj~1.35e1.6 MV/cm), SE emission as well as P-F emission should be the dominant current conduction mechanism for gate injection. SE emission is a field-assisted thermionic emission of electron, the stimulated charge gain enough energy through the interface barrier and finally into the gate, or from the gate into the semiconductor to form the gate leakage current. The SE emission follows the formula expressed by: [33]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 q 4B qE=4pε0 εr 5 ¼ A* T 2 exp4 kB T
(6)
where A* is the effective Richardson constant (A ¼ 120ðm*ox =m0 Þ) (A/cm2K2), T is the absolute temperature, q4B is Al/HfGdO(N) barrier height, E is the electric field in oxide, ε0 and εr are the vacuum dielectric constant and the optical dielectric constant [30], kB is the Boltzmann constant, m0 is the free electron mass, and m*ox is the electron effective mass in HfGdO(N), respectively. Fig. 8(b) plots ln(J/T2) versus the E1/2, the good linear fitting indicates the Schottky emission through the oxide layer. Similar phenomenon has been investigate by Wang et al. for HfGdO/SiO2 on Si deposited by Hf/Gd dual-sputtered method [34]. The intercept of the Schottky plot at the vertical axis can be expressed as:
qf m* intercept ¼ ln A* B ; A ¼ 120 ox kT m0
1=2 m*ox m0 f3B
(9)
The electron relative masses in HfGdO(N) and barrier height at Al/HfGdO(N) interface can be determined by matching the two equations, namely, the intercept of the Schottky plot [Eq. (7)] and the slope of the FN plot [Eq. (9)]. Fig. 9 plots the relationships between m*ox and the q4B for all four samples extracted from the intercept of the Schottky plot and the slope of the FN plot for gate injection. In the case of the F-N tunneling, the barrier height q4B of Al/HfGdO(N) increase with the increasing of m*ox , while for the Schottky emission, the values of q4B show an opposite trend. The m*ox and the q4B of all four samples were obtained by using the mathematical iteration method, as listed in Table 2. The values of (m*ox , q4B) are (0.20 m0, 1.23 eV), (0.18 m0, 1.45 eV), (0.20 m0, 1.21 eV), and (0.25 m0, 1.31 eV) for Al/HfGdON for the S1, S2, S3 and S4 samples, respectively. The discrepancies in m*ox and q4B for S1eS4 samples may be related to characteristics of HfGdO(N) gate dielectrics. Compared 4B values of all samples, it can be noted that S2 sample exhibit highest Schottky barrier height than other samples. So charge require more energy to pass through the gate into the semiconductor, which could correspond to the best interface quality. Based on the analyses above, in the case of gate injection, the current conduct mechanisms are SE emission and P-F emission at lower electric field. At higher field region, FN tunneling is dominant conduction mechanism. However, for substrate injection, F-N tunneling dominates the conduction mechanism at higher electric field. 4. Conclusions
2
JSE
slope ¼ 6:83 107
2205
In summary, the effect of nitrogen incorporation concentration on the interfacial bonding state, electrical characteristics and leakage current conduction mechanisms of MOS capacitors fabricated using sputtering-derived HfGdON as gate dielectrics on Ge has been systemically investigated. Based on XPS analysis of HfGdO(N)/Ge gate stacks, it can be concluded that generation of germanate at the interface between HfGdON film and Ge substrate is suppressed after N doping into HfGdO thin films. Additionally, appropriate amount of N doped into HfGdO can inhibit the
(7)
The intercept is a function of q4B and m*ox . However, the barrier height of Al/HfGdO(N) cannot be determined from the intercept of the Schottky plot alone, unless the electron effective mass is known. In order to obtain both q4B and m*ox at the Al/HfGdO(N) interface, the F-N tunneling current is need to be studied. Fig. 8(c) and (d) plot ln(J/E2) versus the 1/E, the good linear fitting indicates the F-N tunneling very well in higher electric fields (jEjS14 MV/cm) for both substrate injection and gate injection. JFN can be expressed by: [33]
JFN ¼
" pffiffiffiffiffiffiffiffiffiffiffi 3=2 # 4 2m*ox 4B q3 E 2 exp 3ZqE 16p2 Z4B
(8)
4B is the Al/high-k barrier height, m*ox is the electron effective mass in the oxide. The slope of the F-N plot can be expressed as:
Fig. 9. The relationships between the m*ox and the q4B at Al/HfGdO and Al/HfGdON interface extracted from the intercept of the Schottky plot and the slope of the F-N plot for gate injection.
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Table 2 Electron relative effective masses (m*ox =m0 ), barrier heights (q4B), and current conduction mechanism for the Al/HfGdO(N)/Ge MOS capacitors. Samples
S1 S2 S3 S4
m*ox =m0
0.20 0.18 0.20 0.25
Al/HfGdO(N) barrier height q4B (eV)
Conduction mechanism Gate injection
Substrate injection
1.23 1.45 1.21 1.31
SE, SE, SE, SE,
FN FN FN FN
generation of germanate as well as GeO2. Meanwhile, reduction in band gap and valence band offset has been observed after nitrogen incorporation. Electrical characterizations show that HfGdON gate dielectrics with N2 flow rate of 3 sccm exhibits the largest accumulation capacitance, almost disappeared hysteresis, the highest dielectric constant (~29.2), which can be due to the smallest oxygen-vacancy-related interface states and the increased conduction band offset. The conduction mechanism for Al/HfGdO(N)/ Ge is determined by SE emission, P-F emission, and F-N tunneling at gate injection case. However, for substrate injection, F-N tunneling dominates the conduction mechanism at higher electric field. In general, it can be inferred that the content of N in high-k gate dielectrics should be carefully controlled to meet the best performance requirement for future Ge-based CMOS device.
[13] [14]
[15]
[16]
[17]
[18]
Acknowledgements The authors acknowledge the support from National Key Project of Fundamental Research (2013CB632705), National Natural Science Foundation of China (51572002, 51272159, 11474284, 51502005), Anhui Provincial Natural Science Foundation (1608085MA06), and Outstanding Young Scientific Foundation of Anhui University (KJJQ1103) and “211 project” of Anhui University. References [1] G. He, L.Q. Zhu, Z.Q. Sun, Q. Wang, L.D. Zhang, Integrations and challenges of novel high-k gate stacks in advanced CMOS technology, Prog. Mater. Sci. 56 (2011) 475e572. [2] C. Fleischmann, K. Schouteden, M. Muller, P. Honicke, B. Beckhoff, S. Sioncke, H. Boyen, M. Meuris, C.V. Haesendonck, K. Temst, A. Vantomme, Impact of ammonium sulfide solution on electronic properties and ambient stability of germanium surfaces: towards Ge-based microelectronic devices, J. Mater. Chem. C 26 (2013) 4105e4113. [3] I.-K. Oh, K. Kim, Z. Lee, J.G. Song, C.W. Lee, D. Thompson, H.-B.-R. Lee, W.H. Kim, W.J. Maeng, H. Kim, In situ surface cleaning on a Ge substrate using TMA and MgCp2 for HfO2-based gate oxides, J. Mater. Chem. C 3 (2015) 4852e4858. [4] N. Rahim, D. Misra, Role of hydrogen in Ge/HfO2/Al gate stacks subjected to negative bias temperature instability, Appl. Phys. Lett. 92 (2008) 0235111e023511-3. [5] G. He, Z.Q. Sun, S.W. Shi, X.S. Chen, J.G. Lv, L.D. Zhang, Metal-organic chemical vapor deposition of aluminium oxynitride from propylamineedimethylaluminium hydride and oxygen: growth mode dependence and performance optimization, J. Mater. Chem. 22 (2012) 7468e7477. [6] L.G. Wang, Y.H. Xiong, W. Xiao, L. Cheng, J. Du, H. Tu, A.V. Walle, Computational investigation of the phase stability and the electronic properties for Gd doped HfO2, Appl. Phys. Lett. 104 (2014), 201903-201903-4. [7] J. Ko, J. Kim, S.Y. Park, E. Lee, K. Kim, K.-H. Lim, Y.S. Kim, Solution-processed amorphous hafnium-lanthanum oxide gate insulator for oxide thin-film transistors, J. Mater. Chem. C 2 (2014) 1050e1056. [8] C.H. An, M.S. Lee, J.Y. Choi, H. Kim, Change of the trap energy levels of the atomic layer deposited HfLaOx films with different La concentration, Appl. Phys. Lett. 94 (2009), 262901-262901-3. [9] T.M. Pan, C.S. Liao, H.H. Hsu, C.L. Chen, J.D. Lee, K.T. Wang, Excellent frequency dispersion of thin gadolinium oxide high-k gate dielectrics, Appl. Phys. Lett. 87 (2005), 262908-262908-3. [10] J. Robertson, B. Falabretti, Band offsets of highgate oxides on III-V semiconductors, J. Appl. Phys. 100 (2006), 014111-014111-8. [11] Y.B. Losovyj, D. Wooten, J.C. Santana, J.M. An, K.D. Belashchenko, N. Lozova, J. Petrosky, A. Sokolov, J. Tang, W.D. Wang, N. Arulsamy, P.A. Dowben, Comparison of n-type Gd2O3 and Gd-doped HfO2, J. Phys. Condens. Matter 21 (2009), 045602-045602-8. [12] H. Momida, T. Hamada, T. Yamamoto, T. Uda, N. Umezawa, T. Chikyow,
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[23]
[24]
[25]
[26]
[27] [28]
[29]
[30] [31]
[32]
[33]
[34]
PF PF PF PF
and and and and
FN FN FN FN
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Interface quality modulation, band alignment modification and optimization of electrical properties of HfGdO/Ge gate stacks by nitrogen incorporation J. Gao a, b, G. He a, *, Z.B. Fang c, **, J.G. Lv d, ***, M. Liu e, Z.Q. Sun a a
School of Physics and Materials Science, Radiation Detection Materials & Devices Lab, Anhui University, Hefei 230039, China School of Sciences, Anhui University of Science and Technology, Huainan 232001, China Department of Physics, Shaoxing University, Shaoxing 312000, China d Department of Physics and Electronic Engineering, Hefei Normal University, Hefei 230061, China e Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2016 Received in revised form 23 October 2016 Accepted 5 November 2016 Available online 8 November 2016
Effects of nitrogen incorporation on the interfacial chemical bonding states, band alignment, electrical properties and leakage current conduction mechanisms of HfGdO-based metal-oxide-semiconductor (MOS) capacitors has been investigated by UVeVis transmission spectroscopy, X-ray photoemission spectroscopy (XPS), and electrical measurements. XPS results indicate that incorporation of a moderate amount of nitrogen incorporation into HfGdO gate dielectrics can effectively suppress the formation of low-k GeO2 and Hf(Gd)-Ge-O interfacial layer at the interfacial region. Meanwhile, reduction in band gap and valence band offset and increase in conduction band offset have been observed after nitrogen incorporation. Electrical measurements based on MOS capacitors have shown that MOS capacitor with HfGdON/Ge stacked gate dielectric with N2 flow rate of 3 sccm exhibits small gate leakage current (1.08 103 A/cm2 at Vg ¼ 1 V), almost disappeared hysteresis, and large dielectric constant (29.2). The involved leakage current conduction mechanisms for MOS capacitor devices with and without nitrogen incorporation also have been discussed in detail. © 2016 Elsevier B.V. All rights reserved.
Keywords: High-k gate dielectrics Interface quality Band alignment Electrical properties Leakage current mechanism
1. Introduction In order to meet the requirements of high efficiency and low energy consumption of electrical devices, Si-based metal-oxidesemiconductor field-effect-transistor (MOSFET) have been scale down to their physical limit. To further improve the device performance, germanium (Ge) has been studied as a leading candidate to replace silicon (Si) due to its higher carrier mobility and narrower band gap, as compared to Si [1]. However, the absence of high quality MOS interface compared to that of SiO2/Si interface retards the development of Ge-based MOSFETs devices with high performance. Fortunately, recent progress made in the direct
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (G. He), [email protected] (Z.B. Fang), [email protected] (J.G. Lv). http://dx.doi.org/10.1016/j.jallcom.2016.11.068 0925-8388/© 2016 Elsevier B.V. All rights reserved.
deposition of high-k gate dielectrics due to its superior thermodynamic stability when in contact with Ge substrate have reopened the possibility of realizing Ge-based CMOS devices, overcoming its poor interface quality issues. In recent years, HfO2 as gate dielectrics on Ge has been paid more attention to complementary mobility degradation [2,3]. However, their moderate dielectric constants make Hf-based gate dielectrics limited for future CMOS device scaling with an EOT below 1.0 nm [4]. One of the effective ways to increase Hf-based dielectric permittivity is combining it with another much higher dielectric materials. It has been reported that doping of rare earth oxide (REO) into HfO2, such as La2O3, Pr2O3, and Gd2O3, can improve its physical and electrical properties [5e8]. Especially, more attention has been paid to investigate the Gd2O3 incorporated HfO2 high-k gate dielectrics. Gd2O3 has some advantages, such as relatively high dielectric constant, excellent thermal stability, and almost no hysteresis in MOS devices [9]. Additionally, the conduction band offset (CBO) Of Gd2O3 is greater than that of HfO2. Thus, doping Gd2O3 into HfO2 will increase the
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CBO and improve band offset symmetry [10,11]. Meanwhile, doping nitrogen into high-k gate dielectric oxides can suppress oxygen diffusion through high-k oxide effectively, inhibit interfacial reaction, increase crystallization temperature and passivate oxygen vacancy states [12,13]. But incorporation of nitrogen also brings some disadvantages, such as reduction of band gaps, larger leakage current [14]. Xiong et al. have reported the effect of Gd and N incorporation on the band structure and oxygen vacancies of HfO2 gate dielectric films deposited on Si substrates, and confirmed that the oxygen vacancies are greatly suppressed by Gd and N co-doping under the suitable doping amount [15]. Based on previous publication and observations, it can be noted that the co-doping of Gd and N into HfO2may help to extend the application of Hf-based oxides beyond 16 nm and 11 nm technology nodes due to their symmetric band offsets and observably reduced defect states in the band gap [14]. In spite of these improved interfacial and electrical properties of Gd/N-codoped HfO2 gate dielectrics, these investigations are still focused on the Si-based MOS device. Current, quick development in microelectronics area has pushed the application of Ge-based device integrated with Hf-based high-k gate dielectrics. By far, much less work has been devoted to the investigation of Ge-based MOS devices based on Gd/N-codoped HfO2 high-k gate dielectrics. Systematical investigation and observation of interfacial and electrical characteristics of HfO2/Ge gate stack modulated by Gd/N-codoping is necessary. Additionally, the nitrogen incorporation effect of on the band alignments, electrical properties and leakage current conduction mechanisms of HfGdO(N)/Ge gate stack in particular, has not been systematically investigated. As we know, as an important parameter to determine the electrical performance of FETs, the determination of the band alignment of high-k/Ge hetero structures is needed for a complete FETs technology. In current work, more attention has been paid to the investigation of the interfacial chemical bonding states and electrical characteristics of HfGdO(N)/Ge gate stack as a function of nitrogen incorporation content. Additionally, the evolution of the band offset, as well as the leakage current conduction mechanisms of HfGdO and their nitrided derivatives has been discussed in detail. 2. Experimental Sb-doped n-type Ge (100) wafers were cleaned in acetone for 5 min at 75 C to rewove organic contamination followed by ethanol solution, then washed with a mixed solution (NH3$H2O:H2O ¼ 1:4) for 5 min at room temperature, and cleaned in H2O2 solution (H2O2:H2O ¼ 1:5) for 60 s to form oxides layer, which is followed NH3$H2O solution for 5 min to remove the oxides layer, then blown dry with pure N2. After cleaning, ex-situ HfGdO and HfGdON gate dielectric films were deposited by sputtering equipment (JGP-DZS, Chinese Academy of Sciences, Shenyang Scientific Instrument Co., Ltd) using HfGdO ceramic target (atomic ratio of Hf:Gd ¼ 9:1) in a mixed ambient of Ar and N2. The different N incorporated HfGdON films were obtained by changing the flow rate of Ar/N2 mixture to 20:0, 20:3, 20:7, and 20:10 (sccm), which were assigned as S1, S2, S3, and S4, respectively. The base vacuum was 5.0 104 Pa. The RF power and the working pressure were kept at 60 W and 0.5 Pa during the sputtering process. Physical thickness of the gate dielectric layer was measured by spectroscopic ellipsometry (SC630, SANCO Co, Shanghai). About 40-nmthick HfGdO and HfGdON films were deposited on quartz substrate to obtain their optical band gaps (Eg) by transmittance using ultravioletevisible spectrophotometer (UVevis) techniques. The thickness of HfGdO and HfGdON films for X-ray photoelectron spectroscopy (XPS) measurement was about 4 nm. Ex-situ XPS measurements for HfGdON films were performed by using (ESCALAB 250Xi) system, equipped with an Al Ka radiation source
(1486.6 eV) and hemispheric analyzer with a pass energy of 20 eV. All the collected data were corrected using the binding energy of C 1s peak (284.8 eV). Spectral deconvolution was performed by Shirley background subtraction using a Voigt function convoluting the Gaussian and Lorentzian functions. In order to explore the electrical properties, Al/HfGdO(N)/Ge/Al MOS capacitors were fabricated by sputtering a Al top electrode through a shadow mask with an area of 3.14 104 cm2 and the back surfaces of all samples were deposited with a 200-nm-thick Al by sputtering after the back surface oxide stripping to decrease contact resistance. A semiconductor device analyzer (Agilent B1500A) combined with Cascade Probe Station was used for CeV measurement at room temperature. Short circuit and open circuit calibration were performed before real measurements. Additionally, the leakage current properties were measured by B1500A. All the electrical characterization was performed at room temperature in a shielded dark box. 3. Results and discussion 3.1. Interfacial bonding states analysis To investigate the effect of nitrogen incorporation on the chemical states of HfGdO gate dielectrics, Hf 4f, Ge 3d, N 1s and O 1s core-level spectra have been analyzed by XPS, as shown in Fig. 1. Among of them, Fig. 1(a) shows the core level survey spectra of HfGdO(N)/Ge gate stack at a pass energy of 150 eV and a take-off angle of 45 . It can be seen that all characteristic peaks are derived from Hf, Gd, N, O and Ge elements as well as a small amount of C component introduced from thin film growth or air contamination during the measurement, indicating that the asobtained films were HfGdO and HfGdON films. To confirm the nitrogen incorporation into HfGdO gate dielectrics, N 1s core-level XPS spectra of as-deposited HfGdON films with different nitrogen incorporation have been investigated, as demonstrated in the inset in Fig. 1(a). Based on Fig. 1(a), it can be noted that nitrogen incorporated into the HfGdO films has been observed. In the case of the Ge 3d spectra shown in Fig. 1(b), some distinctive sub peaks are identified. The two broad peaks correspond to Ge0 component and a mixture of multiple Ge oxides chemical bonding states, which can be decomposed into four contributions: three are sub-oxides of Ge, i.e., Ge2O (Ge1þ), GeO (Ge2þ), Ge2O3 (Ge3þ), and the one is native oxide GeO2 (Ge4þ). And there is an additional bonding state component, Ge*, i.e., germanate (Hf-Ge-O), between the Ge3þ and Ge4þ components [16]. Compared with S1 sample, it is clearly seen that for sample S2, the peak corresponding to Ge oxides shifts to lower binding energy. Meanwhile, the relative proportion of the Ge oxides increase. The change may be due to the formation of a large amount Ge suboxide with lower binding energies and reduction of germanate with higher binding energy at the interface between HfGdON film and Ge substrate with the incorporation of a small amount of nitrogen. Additionally, the content of native oxide GeO2 for sample S2 almost keeps unchanged compared with sample S1. With the flow rate of N2 increasing from 7 to 10 sccm, the Ge oxides peak shifts to higher binding energy, which can result from the reduction of Ge suboxide and increase of germanate and native oxide GeO2 with higher binding energies. Therefore, it can be concluded that germanate and native oxide can be effectively suppressed after moderate N doping into HfGdO with N2 flow rate of 3. To further identify the effect of nitrogen incorporation on the interfacial chemical bonding state of HfGdO/Ge gate stack more clearly, O 1s core-level spectra have been investigated, as shown in Fig. 1(c). All spectra are deconvoluted into three peaks located at binding energy of 530 eV, 531.1 eV, and 532.1 eV, corresponding to
J. Gao et al. / Journal of Alloys and Compounds 695 (2017) 2199e2206
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Fig. 1. XPS analysis of HfGdO and HfGdON thin films with different nitrogen incorporation concentration (a) Core level XPS survey spectra (Inset is the corresponding N 1s XPS core level spectra.); (b) Ge 3d XPS core level spectra; (c) O 1s XPS core level spectra; (d) Hf 4f XPS core level spectra.
Hf(Gd)-O, Ge-O, and germanate (Ge-Hf(Gd)-O), respectively [17]. From Fig. 1(c), it can be seen that intensity of Ge-Hf(Gd)-O decreases significantly after N doping into HfGdO with N2 flow rate of 3. The change may be due to the incorporation of nitrogen into HfGdO gate dielectrics and suppress the oxygen diffusion at the HfGdON/Ge interface. Meanwhile, the intensity of Ge-O increase, which can be attributed to the formation of a large amount Ge suboxide. However, with the flow rate of N2 increasing from 7 to 10 sccm, increase in the Ge-Hf(Gd)-O content and reduction in the Ge-O content have been detected. So it is obvious to see that germanate interfacial layer could not be suppressed effectively if too much nitrogen is incorporated into high-k gate dielectrics. In order to further confirm the evolution of the interfacial chemical bonding states related with nitrogen incorporation, Hf 4f core-level spectra have been paid attention, as demonstrated in Fig. 1(d). All of Hf 4f spectra are decomposed into two peaks of Hf 4f5/2 and Hf 4f7/2, which can be fitted well by spin orbit splitting energy of about 1.7 eV [18]. The Hf 4f7/2 peak of the S1 sample without doping N lies at 18.0 eV, and those of the S2eS4 samples shift to lower binding energy, which may be due to the reduction of Hf germanate at the interface between HfGdON film and Ge substrate with the incorporation of nitrogen. However, it is worth noting that with the flow rate of N2 increasing from 3 to 10 sccm, the peaks of Hf 4f shift to higher binding energy sides, which may be due to the fact that the formation of Hf germanate can't be inhibited by further increasing the N2 flow rate [19]. Based on above analysis, it can be concluded that appropriate amount of N doped into HfGdO can suppress the formation of germanate and native oxide GeO2. However, too much N doped into HfGdO can promote the generation of germanate and native oxide GeO2. So it is important to control the flow rate of N2 to improve the interfacial properties between HfGdON films and Ge substrates.
3.2. Determination of band gap and band alignment investigation The magnitude of the band offset (BO) between the high-k oxide film and semiconductor is the most fundamental properties for the reliability of MOS devices. In order to obtain the band offset of the interface between HfGdO(N) films with the Ge substrate, the valence-band offset spectra were investigated. To determine the valence band offsets of samples with different flow rate of Ar:N2, the optical band gap (Eg) of all samples were extracted from the UVeVis transmission spectroscopy. Based on the Tauc relation [20], the optical energy band gap (Eg) values of HfGdO(N)/Ge gate stacks are determined by the following equation:
ahn ¼ A hn Eg
1=2
;
(1)
in which A is a constant characteristic of the semiconductor, a, h, and n is absorption coefficient, Planck constant, and frequency of light, respectively. As shown in Fig. 2, the Eg can be obtained by extrapolating the linear part of the (ahn)2 versus hn curves to the energy axis. With the increase of N doping content, the Eg of HfGdON films gradually shifts to lower energy, ranging from 5.85 to 5.42 eV with the N2 flow rate changing from 0 to 10 sccm, respectively. As we know, the band gap is determined by the difference between the valence band maximum and conduction band minimum. The decrease in Eg value could be attributed to the increase of N doping composition in the HfGdON films. For HfGdON films, the valence band maximum is mainly formed by the mixing of N 2p and O 2p states. Since N 2p state locates above the O 2p state, the increase of N doping content leads to the increase of the valence band maximum of HfGdON thin films [15]. Kraut's method has been used to determined the valence-band
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DEC ðHfGdON GeÞ ¼ Eg ðHFGdONÞ DEV ðHfGdON GeÞ Eg ðGeÞ
Fig. 2. Determination of band gaps of HfGdO and HfGdON thin films with different nitrogen concentration.
offset at the HfGdON/Ge interface [21]. According to the Kraut's method, the valence band alignment of HfGdON films on Ge substrates could be determined by measuring the valence band maximum (VBM) difference between HfGdON films and the Ge substrates, as expressed in following relation:
DEV ðHfGdON GeÞ ¼ EV ðHfGdONÞ EV ðGeÞ
(3)
where DEc (HfGdON-Ge) is the conduction band offset, Eg (HfGdON) is the band gap of HfGdON film, and Eg (Ge) is the band gap of the Ge substrate taken as 0.66 eV. By combining the band gap values and DEv values, the schematic energy band alignment of HfGdON/ Ge gate stacks corresponding to different N doping corporation is summarized as shown in Fig. 4. The obtained DEc values are 2.36, 2.41, 2.32, and 2.25 eV for HfGdON films with the N content of 0, 3, 7, and 10 sccm, respectively. According to band offset analysis, it can be concluded that the valence-band offset as well as the conduction-band offset to the Ge substrates of the HfGdON are greater than 1 eV, which makes it suitable as a candidate for high-k gate dielectrics in Ge-based MOS devices. Compared with the sample S1 (without N doping), the DEc of the S2 sample increases and those of the S3 and S4 samples decrease. The increased DEc is considered as an important factor to reduce leakage current effectively for COMS devices. So it can be assumed that by doping the appropriate amount of N into HfGdO, the valence band offset could be increased, which can effectively reduce the leakage current. However, with the amount of N doping into HfGdO increasing from 7 to 10 sccm, the band gap (Eg) continue decreases. Meanwhile, the valence band offset decrease, which would lead to the uncontrollable leakage current and degraded device performance.
(2)
The Ev is determined by using the linear extrapolation method as shown in Fig. 3. The valence band energy Ev(Ge) is located at 0.28 eV. The valence band offset values (DEv) of HfGdON are 2.82, 2.64, 2.54, and 2.50 eV, with the N2 flow rate changing from 0, 3, 5 to 10 sccm, respectively. It is obviously that the values of DEv decrease with the increase of N doping composition into HfGdON. The VB of HfGdO film is mainly determined by O 2p states. After N doping into HfGdO thin films, the contribution of N 2p state, which lies above that of O 2p state, cannot be ignored. As a result, the VB of HfGdON film would raise the valence band maximum and reduce the valence band offset of the films. Moreover, the more N content in the HfGdON thin films, the smaller VB offsets [15]. The conduction band offset (DEc) can be obtained by simply subtracting the valence band offset and the energy gap of the Ge substrate from the band gap of a HfGdON film, as expressed in the following equation:
Fig. 3. The difference between the VBM of HfGdO(N) and Ge substrate with different nitrogen concentration.
3.3. Electrical properties characteristics Fig. 5 demonstrates the high-frequency (1 MHz) CeV characteristics of the Al/HfGdON/Ge MOS capacitors with different Ndoping content. It can be noted that obvious changes have taken
Fig. 4. Schematic band diagram of HfGdO and HfGdON thin films with different nitrogen concentration.
J. Gao et al. / Journal of Alloys and Compounds 695 (2017) 2199e2206
Fig. 5. Capacitanceevoltage (CeV) characteristics of HfGdO and HfGdON films with different nitrogen doping content. The inset is the CeV curve of HfGdON film with the N flow rate of 3 sccm.
place in the CeV curves after nitrogen incorporation, indicating the change in quality of HfGdON films with increase in N content. For S1 sample, an obvious hump exists in the inversion region and a kink exists in depletion region, which can be caused by slow interface states [22]. However, the hump and kink have been observed to disappear after doping a small amount of N (flow rate of N is 3 sccm), indicating that N-doping into HfGdO prevents the formation of slow interface states. But with the N content increasing from 7 to 10 sccm, the hump once again appears in the inversion region, implying that too much nitrogen is not good for the improvement of the interface quality between HfGdON films with Ge substrates. The relative dielectric constants (k) of the four samples are extracted from the accumulation capacitance without taking into account quantum mechanical corrections, which are 27.8, 29.0, 26.9 and 26.5 with the N2 flow rate changing from 0, 3, 5 to 10 sccm, respectively. All of k values agree with those of HfGdO(N) films reported in previous works [23]. In addition, the extracted positive flat band voltage Vfb from CeV curves suggests that there are some negative native defect or trap oxide charge in the oxide layer and/or at the interface [24]. Compared with S1 sample, S2 sample shows an obvious negative Vfb shift, indicating the less negative oxide charges, which can be attributed to either singly and doubly negatively charged interstitial oxygen atoms, or reduction of defect traps in the film and near the interface [25,26]. However, with increased N flow rate, S3 and S4 sample exhibit obvious negative Vfb shift, which indicates the more positive oxide charges exist in the oxide layer and/or at the interface. The k value, Vfb of four samples are listed in Table 1. The leakage current density versus voltage (JeV) curves for Al/ HfGdO(N)/Ge MOS capacitors with voltage range of 3 V ~ 3 V were shown in Fig. 6. It can be noted that the S2 sample processes the smallest leakage density of about 1.08 103 A/cm2 at 1 V, which can be due to the smallest interface state densities and smallest oxide trapped charge at/near interface, as shown in Table 1. Moreover, another plausible explanation for smallest leakage current is the largest DEc confirmed by previous XPS analysis. Based on electrical analysis and XPS analysis, it can be summarized that
Table 1 Parameters of the Al/HfGdO(N)/Ge MOS capacitors extracted from CeV curves. Samples
EOT (nm)
k
Vfb (V)
J (A/cm2)
S1 S2 S3 S4
1.68 1.60 1.76 1.76
27.8 29.2 26.9 26.5
0.39 0.31 0.44 0.52
3.01 1.08 3.57 2.67
103 103 102 101
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Fig. 6. Leakage current density vs gate voltage (JeV) characteristics of HfGdO and HfGdON films with different N doping content.
HfGdON with the N2 flow rate of 3 sccm exhibits the best interfacial quality and best electrical properties. The largest accumulation capacitance and the smallest leakage current in S2 sample can result from the smallest oxygen-vacancy-related interface states and the biggest conduction band offset. Compared with HfO2 films directed deposited on Ge substrates reported in previous works [27e29], the improvement of capacitance voltage hysteresis and leakage current density have been observed in HfGdO(N) samples. There is almost no hysteresis voltage in HfGdO(N) samples [27,28], which is attributed to the doping of Gd [9]. The leakage current density is reduced by four orders of magnitude from the order of 101 A/cm2 to 103 A/cm2 [28]. On the basis of above studies, S2 sample (HfGdON with the N2 flow rate of 3 sccm) with the best performance has been selected to investigate the post-deposition annealing effects on electrical properties. After deposition of HfGdON films, rapid thermal annealing (RTA) process was performed at 300, 400 and 500 C for 60s in a vacuum of 6.3 105 bar, respectively. Then, Al/HfGdON/ Ge/Al MOS capacitors were fabricated by sputtering a Al top electrode with an area of 3.14 104 cm2. The high-frequency (1 MHz) CeV curves and the leakage current density versus voltage (JeV) curves of the Al/HfGdON/Ge MOS as a function of annealing temperatures are shown in Fig. 7. From Fig. 7(a), it can be seen that the CeV curves of annealed samples as well as as-deposited sample are almost no hysteresis. All annealed samples show a continually positive Vfb shift compared to the as-deposited sample with annealing temperature increasing, which indicates the generation of more negative interstitial oxide charges in the oxide layer and/or at the interface during annealing [23]. The relative dielectric constant (k) of the samples extracted from the accumulation capacitance are 29.2, 24.9, 25.6, and 26.7 for the as-deposited and annealed samples from 300 to 500 C, respectively. The values of k decreases after annealing process. The small dielectric constant may be resulted from the microstructure change such as interface property and defects in the film, etc [30,31]. Based on above analysis, it can be drawn that the interface property become worse during annealing process. This conclusion can be further confirmed in the JeV curve shown in Fig. 7(b). From JeV curves, it can be seen that the ASD sample processes the smallest leakage density of about 1.08 103 A/cm2 at 1 V, which can be due to the best interface quality and smallest oxide trapped charge at/near interface. In a words, RAT process in the vacuum does not improve the film quality; on the contrary, annealing process causes deterioration of film properties. Much more work is needed to investigate the effect of the post-annealing temperature
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Fig. 7. (a) Capacitanceevoltage (CeV) characteristics of Al/HfGdON/Ge gate stacks with the N flow rate of 3 sccm annealed at different temperature; (b) Leakage current density vs gate voltage (JeV) characteristics of Al/HfGdON/Ge gate stacks with the N flow rate of 3 sccm annealed at different temperature.
on electrical properties. 3.4. Charge conduction mechanisms in oxides Based on JeV measurements, it has been noted that in HfGdO(N)/Ge gate stacks J under gate injection (V > 0) is much larger than that under substrate injection (V < 0) at same absolute gate voltage, which is attributed to the different energy band structure under different injection mode [32]. JeV curves are different, suggesting that carrier transportation mechanisms are different in the four as-investigated samples. In order to understand the influence of N doping concentration on the carrier transportation mechanisms of HfGdO(N)/Ge gate stacks, several different current conduction mechanisms were explored for both substrate injection and gate injection in current work, including Poole-Frenkle (P-F) emission, Schottky emission (SE), and FowlerNordheim (F-N) tunneling [33]. Fig. 8(a)e(d) show the fitting lines about leakage current
densities (J) as a function of the applies field (E) of HfGdO(N) films with different N-incorporated content. Through analysis, the similar phenomenon can be found in all samples. In the case of gate injection, the main current conduction mechanisms should be P-F emission as well as SE in the region of lower electric fields, and F-N emission in the region of high electric fields, respectively. For substrate injection, the main current conduction mechanism is F-N emission in the region of higher electric fields. The detailed analysis is described in the following section. It can be determined that for gate injection, the dominant current conduction mechanism is P-F emission in the region of lower field region (jEj~1.35e1.6 MV/cm), which correlated with trap and field assistant current. The P-F emission can be expressed as: [33]
JPF
2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 q ft qE=pε0 εox 5 ¼ AE exp4 kB T
(4)
Fig. 8. Leakage current mechanism for the Al/HfGdO(N)/Ge MOS capacitor with different N doping content. (a) Poole-Frenkle (P-F) emission plots for gate injection; (b) Schottky emission (SE) plots for gate injection; (c) Fowler-Nordheim (F-N) tunneling plots for gate injection; (d) Fowler-Nordheim (F-N) tunneling plots for the substrate injection.
J. Gao et al. / Journal of Alloys and Compounds 695 (2017) 2199e2206
in which Ft is the trap energy level below the conduction band edge of the dielectric, εox is the oxide dielectric constant [33]. Similarly, ln(J/E) should be proportional to the E1/2. Fig. 8(a) shows that the plot of ln(J/E) versus the E1/2 is good linear fitting for all samples, which indicates the P-F emission through the oxide layer. The slope of the ln(J/E)-E1/2 curve can be deduced from P-F emission expression as follows:
1 slope ¼ kT
sffiffiffiffiffiffiffiffiffiffiffiffiffiffi q3 pε0 εox
(5)
from which the dielectric constant εox of HfGdO(N) films can be obtained. The dielectric constant εox extracted from the slope of P-F fitting line under gate injection. The values are 11.1, 6.4, 24.5 and 22.6 for the S1, S2, S3 and S4 samples, respectively. Compared with previous results reported, it can be seen that the obtained εox of S1 and S2 samples derive from experimental data, those of S3 and S4 agree with the electric constant obtained from experiments. The results indicate that for S3 and S4 samples, the P-F emission is the denominated current conduction mechanisms for the gate injection in the medium electric field region. For all samples, it has been found from Fig. 8(b) that in the region of lower electric fields (jEj~1.35e1.6 MV/cm), SE emission as well as P-F emission should be the dominant current conduction mechanism for gate injection. SE emission is a field-assisted thermionic emission of electron, the stimulated charge gain enough energy through the interface barrier and finally into the gate, or from the gate into the semiconductor to form the gate leakage current. The SE emission follows the formula expressed by: [33]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 q 4B qE=4pε0 εr 5 ¼ A* T 2 exp4 kB T
(6)
where A* is the effective Richardson constant (A ¼ 120ðm*ox =m0 Þ) (A/cm2K2), T is the absolute temperature, q4B is Al/HfGdO(N) barrier height, E is the electric field in oxide, ε0 and εr are the vacuum dielectric constant and the optical dielectric constant [30], kB is the Boltzmann constant, m0 is the free electron mass, and m*ox is the electron effective mass in HfGdO(N), respectively. Fig. 8(b) plots ln(J/T2) versus the E1/2, the good linear fitting indicates the Schottky emission through the oxide layer. Similar phenomenon has been investigate by Wang et al. for HfGdO/SiO2 on Si deposited by Hf/Gd dual-sputtered method [34]. The intercept of the Schottky plot at the vertical axis can be expressed as:
qf m* intercept ¼ ln A* B ; A ¼ 120 ox kT m0
1=2 m*ox m0 f3B
(9)
The electron relative masses in HfGdO(N) and barrier height at Al/HfGdO(N) interface can be determined by matching the two equations, namely, the intercept of the Schottky plot [Eq. (7)] and the slope of the FN plot [Eq. (9)]. Fig. 9 plots the relationships between m*ox and the q4B for all four samples extracted from the intercept of the Schottky plot and the slope of the FN plot for gate injection. In the case of the F-N tunneling, the barrier height q4B of Al/HfGdO(N) increase with the increasing of m*ox , while for the Schottky emission, the values of q4B show an opposite trend. The m*ox and the q4B of all four samples were obtained by using the mathematical iteration method, as listed in Table 2. The values of (m*ox , q4B) are (0.20 m0, 1.23 eV), (0.18 m0, 1.45 eV), (0.20 m0, 1.21 eV), and (0.25 m0, 1.31 eV) for Al/HfGdON for the S1, S2, S3 and S4 samples, respectively. The discrepancies in m*ox and q4B for S1eS4 samples may be related to characteristics of HfGdO(N) gate dielectrics. Compared 4B values of all samples, it can be noted that S2 sample exhibit highest Schottky barrier height than other samples. So charge require more energy to pass through the gate into the semiconductor, which could correspond to the best interface quality. Based on the analyses above, in the case of gate injection, the current conduct mechanisms are SE emission and P-F emission at lower electric field. At higher field region, FN tunneling is dominant conduction mechanism. However, for substrate injection, F-N tunneling dominates the conduction mechanism at higher electric field. 4. Conclusions
2
JSE
slope ¼ 6:83 107
2205
In summary, the effect of nitrogen incorporation concentration on the interfacial bonding state, electrical characteristics and leakage current conduction mechanisms of MOS capacitors fabricated using sputtering-derived HfGdON as gate dielectrics on Ge has been systemically investigated. Based on XPS analysis of HfGdO(N)/Ge gate stacks, it can be concluded that generation of germanate at the interface between HfGdON film and Ge substrate is suppressed after N doping into HfGdO thin films. Additionally, appropriate amount of N doped into HfGdO can inhibit the
(7)
The intercept is a function of q4B and m*ox . However, the barrier height of Al/HfGdO(N) cannot be determined from the intercept of the Schottky plot alone, unless the electron effective mass is known. In order to obtain both q4B and m*ox at the Al/HfGdO(N) interface, the F-N tunneling current is need to be studied. Fig. 8(c) and (d) plot ln(J/E2) versus the 1/E, the good linear fitting indicates the F-N tunneling very well in higher electric fields (jEjS14 MV/cm) for both substrate injection and gate injection. JFN can be expressed by: [33]
JFN ¼
" pffiffiffiffiffiffiffiffiffiffiffi 3=2 # 4 2m*ox 4B q3 E 2 exp 3ZqE 16p2 Z4B
(8)
4B is the Al/high-k barrier height, m*ox is the electron effective mass in the oxide. The slope of the F-N plot can be expressed as:
Fig. 9. The relationships between the m*ox and the q4B at Al/HfGdO and Al/HfGdON interface extracted from the intercept of the Schottky plot and the slope of the F-N plot for gate injection.
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Table 2 Electron relative effective masses (m*ox =m0 ), barrier heights (q4B), and current conduction mechanism for the Al/HfGdO(N)/Ge MOS capacitors. Samples
S1 S2 S3 S4
m*ox =m0
0.20 0.18 0.20 0.25
Al/HfGdO(N) barrier height q4B (eV)
Conduction mechanism Gate injection
Substrate injection
1.23 1.45 1.21 1.31
SE, SE, SE, SE,
FN FN FN FN
generation of germanate as well as GeO2. Meanwhile, reduction in band gap and valence band offset has been observed after nitrogen incorporation. Electrical characterizations show that HfGdON gate dielectrics with N2 flow rate of 3 sccm exhibits the largest accumulation capacitance, almost disappeared hysteresis, the highest dielectric constant (~29.2), which can be due to the smallest oxygen-vacancy-related interface states and the increased conduction band offset. The conduction mechanism for Al/HfGdO(N)/ Ge is determined by SE emission, P-F emission, and F-N tunneling at gate injection case. However, for substrate injection, F-N tunneling dominates the conduction mechanism at higher electric field. In general, it can be inferred that the content of N in high-k gate dielectrics should be carefully controlled to meet the best performance requirement for future Ge-based CMOS device.
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