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Synthesis and characterization of HfO2 and ZrO2 thin films deposited by plasma assisted reactive pulsed laser deposition at low temperature
Thin Solid Films 518 (2010) 5442–5446
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of HfO2 and ZrO2 thin films deposited by plasma assisted reactive pulsed laser deposition at low temperature W.T. Tang a, Z.F. Ying a,⁎, Z.G. Hu b, W.W. Li b, J. Sun a, N. Xu a, J.D. Wu a,⁎ a Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, People's Republic of China b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, People's Republic of China
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 2 April 2010 Accepted 2 April 2010 Available online 14 April 2010 Keywords: Hafnia Zirconia Thin films Optical property Plasma assisted deposition Low-temperature preparation
a b s t r a c t A plasma assisted reactive pulsed laser deposition process was demonstrated for low-temperature deposition of thin hafnia (HfO2) and zirconia (ZrO2) films from metallic hafnium or zirconium with assistance of an oxygen plasma generated by electron cyclotron resonance microwave discharge. The structure and the interface of the deposited films on silicon were characterized by means of Fourier transform infrared spectroscopy, which reveals the monoclinic phases of HfO2 and ZrO2 in the films with no interfacial SiOx layer between the oxide film and the Si substrate. The optical properties of the deposited films were investigated by measuring the refractive indexes and extinction coefficients with the aid of spectroscopic ellipsometry technique. The films deposited on fused silica plates show excellent transparency from the ultraviolet to near infrared with sharp ultraviolet absorption edges corresponding to direct band gap. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the most important high index materials for optical coatings, group IVB metal oxides, especially hafnium- and zirconium-based oxides hafnia (HfO2) and zirconia (ZrO2), are most attractive due to their transparency over a wide spectral range from the ultraviolet (UV) to the mid-infrared (M-IR), relatively high damage threshold, good thermal and mechanical stabilities [1–4]. HfO2 and ZrO2 are promising for optical coating applications including filters, beamsplitters, anti-reflection coatings and high-reflectivity mirrors [5,6], particularly in the applications where high optical damage thresholds are needed [2,3,7,8]. The optical properties of HfO2 and ZrO2 thin films have been extensively studied [4,7,9–14]. However, there has been a large scatter in the reported values of the optical constants. In addition to the applications in optics, group IVB metal oxides HfO2 and ZrO2 have also potential applications in microelectronics. They are recognized as the possible high-K candidates to replace conventional SiO2 as gate dielectrics in the next generation of complementary metal-oxide-semiconductor (CMOS) devices because of their medium dielectric constants, relatively wide band gaps, and reasonable band gap offsets with silicon, as well as good thermally stability with Si at high temperatures [15–17]. HfO2 and ZrO2 thin films can be deposited by many methods such as electron beam evaporation, chemical vapor deposition, magnetron
⁎ Corresponding authors. E-mail addresses: [email protected] (J.D. Wu), [email protected] (Z.F. Ying). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.04.012
sputtering and pulsed laser deposition [4,9,11,12,14,18–20]. Deposition methods have also been exploited for the preparation of HfO2 and ZrO2 thin films. For applications as gate dielectrics in CMOS, for instance, atomic layer deposition method seems promising for growing HfO2 and ZrO2 films since it shows its unique ability in depositing ultrathin films with excellent conformity and uniformity over large areas [13,21]. Although significant progress has been made in the deposition of HfO2 and ZrO2 films, high growth temperatures are usually required to form the desired oxide phase. However, high temperature processes will introduce undesirable effects such as thermal strain and defects in the deposited films, phase segregation and chemical reactions at the interface, and substrate deterioration. For instance, when HfO2 and ZrO2 films are deposited on silicon, the oxidation of silicon surface usually occurs by the oxygen-containing environment during high-temperature deposition of oxides, resulting in the formation of a SiOx interfacial layer between the oxide film and the silicon substrate [22,23]. When being used as the dielectrics in CMOS, the presence of SiOx layer limits the minimum achievable equivalent oxide thickness [16]. In this respect, low-temperature deposition method is preferable for controlling interface reactions and eliminating the oxidation of silicon surface. We have recently developed a plasma assisted reactive pulsed laser deposition (PARPLD) method for low-temperature reactive deposition of compound films by combining pulsed laser ablation and electron cyclotron resonance (ECR) microwave discharge [24,25]. In the PARPLD process, the reactive gaseous species activated by discharge in the ECR plasma react at high rates with the energetic gaseous
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species ablated from the target, at the same time the low-energy plasma stream which concurrently bombards the growing film assists film growth. The combination of the assistance of the reactive and energetic plasma with the energetic gaseous precursors produced in the PARPLD process allows the low-temperature growth of compound films. In the present work, this PARPLD method was used to deposit HfO2 and ZrO2 films at low temperature by pulsed laser ablation of a metallic hafnium or zirconium target with the assistance of ECR oxygen plasma. The deposited HfO2 and ZrO2 films were characterized by Fourier transform infrared (FTIR) spectroscopy for the determination of oxide phases and the characterization of film structure. The optical properties of the films were studied over the ultraviolet– visible–near infrared (UV–Vis–NIR) wavelength range by determining the optical constants refractive index (n) and extinction coefficient (k) and measuring the optical transmittance. 2. Experimental HfO2 and ZrO2 thin films were deposited by PARPLD from metallic hafnium or zirconium with the assistance of oxygen plasma generated by ECR microwave discharge of pure O2 gas. Home-made PARPLD equipment combined a PLD system with an ECR plasma source was used for film deposition. The details of the PARPLD equipment have been described elsewhere [24,25]. In the present work, high purity (N99.999%) O2 gas was used as the working gas in the ECR plasma source for the generation of oxygen plasma. After the discharge chamber and the deposition chamber were evacuated to a base pressure of ∼ 10− 4 Pa, O2 gas was introduced into the discharge chamber at a working pressure of 4 × 10− 2 Pa with a steady gas flow of about 16 sccm. The discharge chamber filled with the steady working O2 gas flow was first set at the ECR state. The discharge of the O2 gas was ignited by the ablation of the target by the first laser pulse and then a continuous oxygen plasma was maintained during film deposition. That is to say that the generation of oxygen plasma which served as the reactive environment and the deposition of oxide films started at the same time to eliminate the oxidation of the Si substrate surface before film deposition. The oxygen plasma was introduced into the deposition chamber where target ablation and film deposition were performed. The starting material was metallic hafnium (N99.95% in purity) or zirconium (N99.99% in purity). The target made of pure hafnium or zirconium was mounted on a rotating stage in the deposition chamber and kept rotating to prevent crater formation. Laser pulses from the second harmonic of a Q-switched Nd: YAG laser (wavelength: 532 nm; pulse duration: 5 ns; repetition rate: 10 Hz) were used to ablate the target after being focused by a plane-convex lens. The focused laser beam with a spot size of about 1.2 mm2 was incident on the target surface at an angle of 45° and the laser energy used to ablate the target was set at 40 mJ/pulse. Polished single crystalline Si (100) wafers with a resistivity of 5 Ω cm and double-side polished fused silica (UV-grade) plates were used as substrates. The Si wafers were supersonically cleaned in alcohol and dipped in 10% HF solution to remove surface contaminants and natural oxide layer. The silica plates were supersonically cleaned in alcohol and rinsed by ionized water. The substrate was positioned parallel to the target with a distance of 40 mm. The substrate was also kept rotating for film uniformity. All films were deposited for 60 min with film thickness of about 100 nm. During the deposition, the growing film was simultaneously bombarded by the oxygen plasma stream. No auxiliary heater was applied to the substrate. However, the bombardment by the low-energy plasma stream slightly heated the substrate whose temperature rose to no more than 80 °C. No post-deposition treatments were performed for the deposited films. FTIR spectroscopy in transmittance mode in the spectral range of 400–4000 cm− 1 was used to analyze the vibrational modes within the as-deposited films for structure characterization. FTIR measurement was carried out using a Nicolet FTIR spectrometer (NEXUS 670).
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The optical properties of the deposited thin films were investigated using variable angle spectroscopic ellipsometry (SE) technique (SC630UVN) over the wavelength range 260–1700 nm with a spectral resolution of 2 nm. The optical constants of the films were derived by fitting the recorded SE spectra. With a double beam ultraviolet– infrared spectrophotometer (Perkin-Elmer Lambda 950), the optical properties were also studied by measuring transmission spectra from 190 to 2650 nm for the films deposited on transparent fused silica substrates with the deposition conditions remaining the same as those for depositing films on Si substrates. All the ex situ measurements were done in air at room temperature. 3. Results and discussion For the monoclinic structure of HfO2 and ZrO2, 15 modes (8Au + 7Bu) have been theoretically predicted IR active and some of them observed in publications [26–29]. In the present work, FTIR characterization in the wavenumber range from 400 to 4000 cm− 1 confirmed the formation of monoclinic HfO2 and ZrO2 thin films by PARPLD. The recorded FTIR spectrum in the transmission mode for an as-deposited HfO2 film on a Si substrate is displayed in Fig. 1 after correcting the Si background. All vibrational features expected for pure HfO2 are located below ∼ 800 cm− 1, revealing the crystalline structure of the HfO2 films which is predominantly the monoclinic phase. While most as-deposited HfO2 thin films prepared by various methods were reported amorphous and their FTIR spectra are featureless [22,23,29,30], the FTIR spectra of our as-deposited films clearly exhibits three sharp absorption bands at 417, 516 and 605 cm− 1 together with a broad one centered at about 730 cm− 1. The three sharp absorption bands correspond to specific HfO vibrational modes. They are all assigned to the monoclinic phase of HfO2 [29]. The broad band most probably originates from an amorphous HfO2 matrix. On this basis, it would appear that the as-deposited HfO2 films by PARPLD at low temperature in the present work contain predominantly monoclinic HfO2 phase with a small amount of amorphous HfO2. It is known that depositing an oxide film on a Si substrate often results in the formation of an interfacial SiO2 layer mainly originating from the oxidation of Si substrate due to the higher negative enthalpy of formation of SiO2 (−ΔHSiO2=910.7 kJ/mol) [31]. In some publications reported for the FTIR analysis of HfO2 thin films on Si substrates, there usually appeared some bands attributed to Si–O bond absorptions, indicating the existence of an interfacial SiOx layer between the HfO2 film and the Si substrate [22,23]. FTIR analysis was also used in our work to monitor the formation of any interfacial SiOx layer. No
Fig. 1. FTIR spectra of HfO2 (black) and ZrO2 (blue) films deposited on Si substrates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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bands attributed to Si–O–Si vibrational modes were observed in the FTIR measurements for our PARPLD HfO2 films, as is clearly shown in Fig. 1. The absence of any absorption bands attributed to Si–O related modes indicates that the PARPLD HfO2 films deposited on Si substrates are SiOx-free in the interface. The clean film/substrate interface is indebted to the PARPLD process in which the oxygen plasma was ignited no sooner than the beginning of film deposition and the HfO2 films were deposited at a temperature lower than 80 °C, hence Si surface oxidation before and during film deposition should be minimized or even avoided. Similar results were obtained by FTIR analysis for the structure characterization of the deposited ZrO2 thin films and the interface between the ZrO2 film and the Si substrate. A typical FTIR spectrum recorded for a ZrO2 film deposited on a Si substrate is also shown in Fig. 1 which is consistent with that of monoclinic ZrO2 with absorption bands at 410, 506, 590 and 720 cm− 1 [29]. The FTIR spectrum of the ZrO2 film is very similar to that of the HfO2 film, except that its absorption band at 720 cm− 1 is much stronger and narrower than the counterpart one centered at about 730 cm− 1 attributed to the amorphous HfO2 matrix, indicating that the deposited ZrO2 films have a better crystalline structure than the HfO2 films. We also noticed that no absorption bands corresponding to Si–O related modes were observed for the ZrO2 films deposited on Si substrates. The optical properties of the HfO2 and ZrO2 films deposited on Si substrates were studied using spectroscopic ellipsometry performed in the wavelength range from 260 to1700 nm. Refractive indexes (n) and extinction coefficients (k) of the films were determined by fitting the SE data using the Cauchy–Urbach dispersion model, together with the determination of film thickness (∼100 nm). Fig. 2 shows the determined dispersion (n–λ) curve of the HfO2 films. It can be seen that the refractive indexes exhibit a strong dispersion and decrease monotonically with increasing wavelength. It is noticed that the refractive index of the HfO2 films at 600 nm is 2.12, which is somewhat larger than the bulk refractive index 2.08 at 600 nm [12,32]. The k–λ curve of the deposited HfO2 films is also presented in Fig. 2. One can see that the extinction coefficients of the HfO2 are very small (b10− 4) over a wide wavelength range from the visible to the NIR, suggesting the realization of high-quality HfO2 film in terms of optical properties. Even in the UV region down to 350 nm, the HfO2 film has extinction coefficients near zero (∼ 10− 2). The deposited ZrO2 films were found to have similar n–λ dispersion but larger refractive indexes than those of the HfO2 films, as shown in Fig. 3. The values of refractive indexes of the ZrO2 films were determined to be in the range 2.16–
Fig. 2. Spectral dependence of refractive index (black) and extinction coefficient (blue) of HfO2 films deposited on Si substrates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Spectral dependence of refractive index of ZrO2 films (blue) deposited on Si substrates in comparison with that of HfO2 films (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.52. However, the extinction coefficients of the ZrO2 films are too small to be determined by fitting the SE data using the same mode. For further optical characterization in the region from the UV to the NIR, the transmittance of the HfO2 and ZrO2 films deposited on transparent fused silica substrates was measured in the UV–Vis–NIR range. Fig. 4 illustrates the transmission spectra ranging from 190 to 2650 nm for the HfO2 and ZrO2 films. It can be seen that both the HfO2 and ZrO2 films shows excellent transparency with optical transmittances of about 80% in a wide region. Their transmittances are even higher in the NIR with no obvious variations, above 85% for the HfO2 films and above 90% for the ZrO2 films, respectively. On the other hand, both the deposited HfO2 and ZrO2 films are still transparent down to wavelength near 250 nm and show a sharp ultraviolet absorption edge at about 200 nm. Extrapolation of the linear parts of the transmission spectra in the short wavelength range yields the fundamental absorption edge λcutoff, which gives the λcutoff near 190 nm, the short wavelength limit of the used spectrophotometer, for the HfO2 and smaller than 190 nm for the ZrO2 films, respectively. The optical absorption near the short wavelength absorption edge is dominated by the band gap transition of the semiconductor. Hence the optical band gap energy Eg can be determined from the absorption spectra by extrapolating the linear parts of the (αhν)2 vs. hν or (αhν)1/2
Fig. 4. Transmission spectra of HfO2 (blue) and ZrO2 (black) films deposited on fused silica plates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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vs. hν plots to (αhν) = 0 depending on the structure of the semiconductor, where α and hν are the absorption coefficient and photon energy, respectively [33]. The square function is valid near a direct band gap hence the linear part of the former plot is used to determine direct gaps, while indirect band gaps usually meet square root relation (called the Tauc law) hence can be determined using the latter plot. The Tauc law is also valid for amorphous films [34]. Both the (αhν)2 vs. hν and (αhν)1/2 vs. hν plots have been used to determine the optical bands gap of HfO2 and ZrO2 films. No linear parts were found by Aarik et al. in the (αhν)2 vs. hν curve for their HfO2 thin films grown by atomic layer deposition at 500 K, by contrast, the (αhν)2 vs. hν curves of the films grown at 570–1200 K showed linear parts in a narrow energy range from which the films were estimated to have gap energies of 5.68– 5.72 eV [13]. From the (αhν)2 vs. hν plots for linear regression, Balog et al. determined the band gap energy Eg of 5.65 eV for their monoclinic HfO2 films grown by chemical vapor deposition [9]. Also using the same plot corresponding to the square function, Kosacki et al. estimated the band gap of ZrO2:Y films [10], whereas Kato et al. used the (αhν)1/2 vs. hν plot to estimate the band gap Eg of HfxSi1 − xOy and ZrxSi1 − x Oy [11]. For our HfO2 and ZrO2 films, it appears that the (αhν)2 vs. hν plot rather than the other one is suitable for band gap determination, as shown in Figs. 5 and 6, revealing the direct band and non-amorphous structure of the films. Linear dependence of the (αhν)2 on hν and its extrapolation to (αhν)2 = 0 gives the values of Eg = 5.65 eV and 5.96 eV for the PARPLD HfO2 and ZrO2 films, respectively. By comparing Fig. 5(a) with Fig. 6(a), however, it can be seen that the (αhν)2 vs. hν curve near band gap shows a good linearity for the ZrO2 films, while for the HfO2 films two distinct linear relations exist, suggesting the existence of two different absorption processes in the HfO2 films. The intercepts of the two lines with the photon energy axis (Eg1 and Eg2) represent the onsets for the two different absorption processes. Al-Kuhaili has also observed two linear relations in the (αhν)1/2 vs. hν curve for amorphous HfO2 films deposited by electron beam evaporation and attributed them to the intrinsic absorption of the oxide and the absorption involving some
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Fig. 6. Dependencies of (a) (αhν)2 on photon energy hν and (b) (αhν)1/2on photon energy hν, and determination of the band gap energy for ZrO2 films using (αhν)2 vs. hν plot.
defect states [4]. For our HfO2 films, the lower energy Eg1 = 5.65 eV is attributed to the intrinsic absorption of the oxide, however, at present we could not give an unambiguous explanation for the origin responsible for the other phonon energy Eg2 = 6.07 eV, a rather large energy value. From the (αhν)2 vs. hν plots presented in Fig. 5(a) and Fig. 6(a), in addition, one may speculate that the ZrO2 films have a better monoclinic structure than the HfO2 films, consistent with the observation obtained by the FTIR analysis for the HfO2 and ZrO2 films as described earlier. 4. Conclusions
Fig. 5. Dependencies of (a) (αhν)2 on photon energy hν and (b) (αhv)1/2on photon energy hν, and determination of the band gap energy for HfO2 films using (αhν)2 vs. hν plot.
Monoclinic HfO2 and ZrO2 thin films were depostited at low temperature from metallic hafnium or zirconium. The assistance of the reactive and energetic ECR oxygen plasma and the energetic gaseous precursors produced in the PARPLD process allowed the lowtemperature growth of oxides. The FTIR analysis of the deposited films revealed the characteristics of monoclinic phases of HfO2 and ZrO2, while no Si–O related modes were observed. The low-temperature growth and the concurrence of the ignition of oxygen plasma and the beginning of film deposition minimized the surface oxidation of Si substrates before and during film deposition, hence eliminated the possibility of the formation of interfacial SiOx layer between the oxide film and the Si substrate. The optical properties of the films on silicon substrates were studied by spectroscopic ellipsometry in the wavelength range 260–1700 nm and the optical constants were determined from the curve fitting of the ellipsometric data. The HfO2 and ZrO2 films deposited on fused silica plates show high transparency in the UV–Vis–NIR region with nearly constant transmittances above 85% in the NIR and sharp ultraviolet absorption edges near 200 nm. From the (αhν)2 vs. hν plots, the films were determined to have direct band gap energies of 5.65 eV for HfO2 and 5.96 eV for ZrO2, respectively.
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Acknowledgements The authors would like to acknowledge the support from the National Natural Science Foundation of China under the Contract No. 10875029. Shanghai AM Research and Development Foundation (No. 07SA11) and Shanghai Municipal Commission of Science and Technology (No. 08520706100) are also acknowledged. References [1] M.R. Kozlowski, in: F. Flory (Ed.), Thin Films for Optical Systems, Dekker, New York, 1996, p. 521. [2] C. Giuri, M.R. Perrone, V. Piccinno, Appl. Opt. 36 (1997) 1143. [3] J.D.T. Kruschwitz, W.T. Pawlewicz, Appl. Opt. 36 (1997) 2157. [4] M.F. Al-Kuhaili, Opt. Mater. 27 (2004) 383. [5] M. Zukic, D.G. Torr, J.F. Spann, M.R. Torr, Appl. Opt. 29 (1990) 4284. [6] S.M. Edlou, A. Smajkiewicz, G.A. Al-Jumaily, Appl. Opt. 32 (1993) 5601. [7] M. Gilo, N. Croitoru, Thin Solid Films 350 (1999) 203. [8] M. Alvisi, M. Di Giulio, S.G. Marrone, M.R. Perrone, M.L. Protopapa, A. Valentini, L. Vasanelli, Thin Solid Films 358 (2000) 250. [9] M. Balog, M. Schieber, M. Michman, S. Patai, Thin Solid Films 41 (1977) 247. [10] I. Kosacki, V. Petrovsky, H.U. Anderson, Appl. Phys. Lett. 74 (1999) 341. [11] H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K.S. Seol, Y. Ohki, J. Appl. Phys. 92 (2002) 1106. [12] H. Hu, C. Zhu, Y.F. Lu, Y.H. Wu, T. Liew, M.F. Li, B.J. Cho, W.K. Choi, N. Yakovlev, J. Appl. Phys. 94 (2003) 551. [13] J. Aarik, H. Mändar, M. Kirm, L. Pung, Thin Solid Films 466 (2004) 41. [14] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of HfO2 and ZrO2 thin films deposited by plasma assisted reactive pulsed laser deposition at low temperature W.T. Tang a, Z.F. Ying a,⁎, Z.G. Hu b, W.W. Li b, J. Sun a, N. Xu a, J.D. Wu a,⁎ a Key Laboratory of Micro and Nano Photonic Structures, Ministry of Education, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, People's Republic of China b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, People's Republic of China
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 2 April 2010 Accepted 2 April 2010 Available online 14 April 2010 Keywords: Hafnia Zirconia Thin films Optical property Plasma assisted deposition Low-temperature preparation
a b s t r a c t A plasma assisted reactive pulsed laser deposition process was demonstrated for low-temperature deposition of thin hafnia (HfO2) and zirconia (ZrO2) films from metallic hafnium or zirconium with assistance of an oxygen plasma generated by electron cyclotron resonance microwave discharge. The structure and the interface of the deposited films on silicon were characterized by means of Fourier transform infrared spectroscopy, which reveals the monoclinic phases of HfO2 and ZrO2 in the films with no interfacial SiOx layer between the oxide film and the Si substrate. The optical properties of the deposited films were investigated by measuring the refractive indexes and extinction coefficients with the aid of spectroscopic ellipsometry technique. The films deposited on fused silica plates show excellent transparency from the ultraviolet to near infrared with sharp ultraviolet absorption edges corresponding to direct band gap. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the most important high index materials for optical coatings, group IVB metal oxides, especially hafnium- and zirconium-based oxides hafnia (HfO2) and zirconia (ZrO2), are most attractive due to their transparency over a wide spectral range from the ultraviolet (UV) to the mid-infrared (M-IR), relatively high damage threshold, good thermal and mechanical stabilities [1–4]. HfO2 and ZrO2 are promising for optical coating applications including filters, beamsplitters, anti-reflection coatings and high-reflectivity mirrors [5,6], particularly in the applications where high optical damage thresholds are needed [2,3,7,8]. The optical properties of HfO2 and ZrO2 thin films have been extensively studied [4,7,9–14]. However, there has been a large scatter in the reported values of the optical constants. In addition to the applications in optics, group IVB metal oxides HfO2 and ZrO2 have also potential applications in microelectronics. They are recognized as the possible high-K candidates to replace conventional SiO2 as gate dielectrics in the next generation of complementary metal-oxide-semiconductor (CMOS) devices because of their medium dielectric constants, relatively wide band gaps, and reasonable band gap offsets with silicon, as well as good thermally stability with Si at high temperatures [15–17]. HfO2 and ZrO2 thin films can be deposited by many methods such as electron beam evaporation, chemical vapor deposition, magnetron
⁎ Corresponding authors. E-mail addresses: [email protected] (J.D. Wu), [email protected] (Z.F. Ying). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.04.012
sputtering and pulsed laser deposition [4,9,11,12,14,18–20]. Deposition methods have also been exploited for the preparation of HfO2 and ZrO2 thin films. For applications as gate dielectrics in CMOS, for instance, atomic layer deposition method seems promising for growing HfO2 and ZrO2 films since it shows its unique ability in depositing ultrathin films with excellent conformity and uniformity over large areas [13,21]. Although significant progress has been made in the deposition of HfO2 and ZrO2 films, high growth temperatures are usually required to form the desired oxide phase. However, high temperature processes will introduce undesirable effects such as thermal strain and defects in the deposited films, phase segregation and chemical reactions at the interface, and substrate deterioration. For instance, when HfO2 and ZrO2 films are deposited on silicon, the oxidation of silicon surface usually occurs by the oxygen-containing environment during high-temperature deposition of oxides, resulting in the formation of a SiOx interfacial layer between the oxide film and the silicon substrate [22,23]. When being used as the dielectrics in CMOS, the presence of SiOx layer limits the minimum achievable equivalent oxide thickness [16]. In this respect, low-temperature deposition method is preferable for controlling interface reactions and eliminating the oxidation of silicon surface. We have recently developed a plasma assisted reactive pulsed laser deposition (PARPLD) method for low-temperature reactive deposition of compound films by combining pulsed laser ablation and electron cyclotron resonance (ECR) microwave discharge [24,25]. In the PARPLD process, the reactive gaseous species activated by discharge in the ECR plasma react at high rates with the energetic gaseous
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species ablated from the target, at the same time the low-energy plasma stream which concurrently bombards the growing film assists film growth. The combination of the assistance of the reactive and energetic plasma with the energetic gaseous precursors produced in the PARPLD process allows the low-temperature growth of compound films. In the present work, this PARPLD method was used to deposit HfO2 and ZrO2 films at low temperature by pulsed laser ablation of a metallic hafnium or zirconium target with the assistance of ECR oxygen plasma. The deposited HfO2 and ZrO2 films were characterized by Fourier transform infrared (FTIR) spectroscopy for the determination of oxide phases and the characterization of film structure. The optical properties of the films were studied over the ultraviolet– visible–near infrared (UV–Vis–NIR) wavelength range by determining the optical constants refractive index (n) and extinction coefficient (k) and measuring the optical transmittance. 2. Experimental HfO2 and ZrO2 thin films were deposited by PARPLD from metallic hafnium or zirconium with the assistance of oxygen plasma generated by ECR microwave discharge of pure O2 gas. Home-made PARPLD equipment combined a PLD system with an ECR plasma source was used for film deposition. The details of the PARPLD equipment have been described elsewhere [24,25]. In the present work, high purity (N99.999%) O2 gas was used as the working gas in the ECR plasma source for the generation of oxygen plasma. After the discharge chamber and the deposition chamber were evacuated to a base pressure of ∼ 10− 4 Pa, O2 gas was introduced into the discharge chamber at a working pressure of 4 × 10− 2 Pa with a steady gas flow of about 16 sccm. The discharge chamber filled with the steady working O2 gas flow was first set at the ECR state. The discharge of the O2 gas was ignited by the ablation of the target by the first laser pulse and then a continuous oxygen plasma was maintained during film deposition. That is to say that the generation of oxygen plasma which served as the reactive environment and the deposition of oxide films started at the same time to eliminate the oxidation of the Si substrate surface before film deposition. The oxygen plasma was introduced into the deposition chamber where target ablation and film deposition were performed. The starting material was metallic hafnium (N99.95% in purity) or zirconium (N99.99% in purity). The target made of pure hafnium or zirconium was mounted on a rotating stage in the deposition chamber and kept rotating to prevent crater formation. Laser pulses from the second harmonic of a Q-switched Nd: YAG laser (wavelength: 532 nm; pulse duration: 5 ns; repetition rate: 10 Hz) were used to ablate the target after being focused by a plane-convex lens. The focused laser beam with a spot size of about 1.2 mm2 was incident on the target surface at an angle of 45° and the laser energy used to ablate the target was set at 40 mJ/pulse. Polished single crystalline Si (100) wafers with a resistivity of 5 Ω cm and double-side polished fused silica (UV-grade) plates were used as substrates. The Si wafers were supersonically cleaned in alcohol and dipped in 10% HF solution to remove surface contaminants and natural oxide layer. The silica plates were supersonically cleaned in alcohol and rinsed by ionized water. The substrate was positioned parallel to the target with a distance of 40 mm. The substrate was also kept rotating for film uniformity. All films were deposited for 60 min with film thickness of about 100 nm. During the deposition, the growing film was simultaneously bombarded by the oxygen plasma stream. No auxiliary heater was applied to the substrate. However, the bombardment by the low-energy plasma stream slightly heated the substrate whose temperature rose to no more than 80 °C. No post-deposition treatments were performed for the deposited films. FTIR spectroscopy in transmittance mode in the spectral range of 400–4000 cm− 1 was used to analyze the vibrational modes within the as-deposited films for structure characterization. FTIR measurement was carried out using a Nicolet FTIR spectrometer (NEXUS 670).
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The optical properties of the deposited thin films were investigated using variable angle spectroscopic ellipsometry (SE) technique (SC630UVN) over the wavelength range 260–1700 nm with a spectral resolution of 2 nm. The optical constants of the films were derived by fitting the recorded SE spectra. With a double beam ultraviolet– infrared spectrophotometer (Perkin-Elmer Lambda 950), the optical properties were also studied by measuring transmission spectra from 190 to 2650 nm for the films deposited on transparent fused silica substrates with the deposition conditions remaining the same as those for depositing films on Si substrates. All the ex situ measurements were done in air at room temperature. 3. Results and discussion For the monoclinic structure of HfO2 and ZrO2, 15 modes (8Au + 7Bu) have been theoretically predicted IR active and some of them observed in publications [26–29]. In the present work, FTIR characterization in the wavenumber range from 400 to 4000 cm− 1 confirmed the formation of monoclinic HfO2 and ZrO2 thin films by PARPLD. The recorded FTIR spectrum in the transmission mode for an as-deposited HfO2 film on a Si substrate is displayed in Fig. 1 after correcting the Si background. All vibrational features expected for pure HfO2 are located below ∼ 800 cm− 1, revealing the crystalline structure of the HfO2 films which is predominantly the monoclinic phase. While most as-deposited HfO2 thin films prepared by various methods were reported amorphous and their FTIR spectra are featureless [22,23,29,30], the FTIR spectra of our as-deposited films clearly exhibits three sharp absorption bands at 417, 516 and 605 cm− 1 together with a broad one centered at about 730 cm− 1. The three sharp absorption bands correspond to specific HfO vibrational modes. They are all assigned to the monoclinic phase of HfO2 [29]. The broad band most probably originates from an amorphous HfO2 matrix. On this basis, it would appear that the as-deposited HfO2 films by PARPLD at low temperature in the present work contain predominantly monoclinic HfO2 phase with a small amount of amorphous HfO2. It is known that depositing an oxide film on a Si substrate often results in the formation of an interfacial SiO2 layer mainly originating from the oxidation of Si substrate due to the higher negative enthalpy of formation of SiO2 (−ΔHSiO2=910.7 kJ/mol) [31]. In some publications reported for the FTIR analysis of HfO2 thin films on Si substrates, there usually appeared some bands attributed to Si–O bond absorptions, indicating the existence of an interfacial SiOx layer between the HfO2 film and the Si substrate [22,23]. FTIR analysis was also used in our work to monitor the formation of any interfacial SiOx layer. No
Fig. 1. FTIR spectra of HfO2 (black) and ZrO2 (blue) films deposited on Si substrates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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bands attributed to Si–O–Si vibrational modes were observed in the FTIR measurements for our PARPLD HfO2 films, as is clearly shown in Fig. 1. The absence of any absorption bands attributed to Si–O related modes indicates that the PARPLD HfO2 films deposited on Si substrates are SiOx-free in the interface. The clean film/substrate interface is indebted to the PARPLD process in which the oxygen plasma was ignited no sooner than the beginning of film deposition and the HfO2 films were deposited at a temperature lower than 80 °C, hence Si surface oxidation before and during film deposition should be minimized or even avoided. Similar results were obtained by FTIR analysis for the structure characterization of the deposited ZrO2 thin films and the interface between the ZrO2 film and the Si substrate. A typical FTIR spectrum recorded for a ZrO2 film deposited on a Si substrate is also shown in Fig. 1 which is consistent with that of monoclinic ZrO2 with absorption bands at 410, 506, 590 and 720 cm− 1 [29]. The FTIR spectrum of the ZrO2 film is very similar to that of the HfO2 film, except that its absorption band at 720 cm− 1 is much stronger and narrower than the counterpart one centered at about 730 cm− 1 attributed to the amorphous HfO2 matrix, indicating that the deposited ZrO2 films have a better crystalline structure than the HfO2 films. We also noticed that no absorption bands corresponding to Si–O related modes were observed for the ZrO2 films deposited on Si substrates. The optical properties of the HfO2 and ZrO2 films deposited on Si substrates were studied using spectroscopic ellipsometry performed in the wavelength range from 260 to1700 nm. Refractive indexes (n) and extinction coefficients (k) of the films were determined by fitting the SE data using the Cauchy–Urbach dispersion model, together with the determination of film thickness (∼100 nm). Fig. 2 shows the determined dispersion (n–λ) curve of the HfO2 films. It can be seen that the refractive indexes exhibit a strong dispersion and decrease monotonically with increasing wavelength. It is noticed that the refractive index of the HfO2 films at 600 nm is 2.12, which is somewhat larger than the bulk refractive index 2.08 at 600 nm [12,32]. The k–λ curve of the deposited HfO2 films is also presented in Fig. 2. One can see that the extinction coefficients of the HfO2 are very small (b10− 4) over a wide wavelength range from the visible to the NIR, suggesting the realization of high-quality HfO2 film in terms of optical properties. Even in the UV region down to 350 nm, the HfO2 film has extinction coefficients near zero (∼ 10− 2). The deposited ZrO2 films were found to have similar n–λ dispersion but larger refractive indexes than those of the HfO2 films, as shown in Fig. 3. The values of refractive indexes of the ZrO2 films were determined to be in the range 2.16–
Fig. 2. Spectral dependence of refractive index (black) and extinction coefficient (blue) of HfO2 films deposited on Si substrates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Spectral dependence of refractive index of ZrO2 films (blue) deposited on Si substrates in comparison with that of HfO2 films (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.52. However, the extinction coefficients of the ZrO2 films are too small to be determined by fitting the SE data using the same mode. For further optical characterization in the region from the UV to the NIR, the transmittance of the HfO2 and ZrO2 films deposited on transparent fused silica substrates was measured in the UV–Vis–NIR range. Fig. 4 illustrates the transmission spectra ranging from 190 to 2650 nm for the HfO2 and ZrO2 films. It can be seen that both the HfO2 and ZrO2 films shows excellent transparency with optical transmittances of about 80% in a wide region. Their transmittances are even higher in the NIR with no obvious variations, above 85% for the HfO2 films and above 90% for the ZrO2 films, respectively. On the other hand, both the deposited HfO2 and ZrO2 films are still transparent down to wavelength near 250 nm and show a sharp ultraviolet absorption edge at about 200 nm. Extrapolation of the linear parts of the transmission spectra in the short wavelength range yields the fundamental absorption edge λcutoff, which gives the λcutoff near 190 nm, the short wavelength limit of the used spectrophotometer, for the HfO2 and smaller than 190 nm for the ZrO2 films, respectively. The optical absorption near the short wavelength absorption edge is dominated by the band gap transition of the semiconductor. Hence the optical band gap energy Eg can be determined from the absorption spectra by extrapolating the linear parts of the (αhν)2 vs. hν or (αhν)1/2
Fig. 4. Transmission spectra of HfO2 (blue) and ZrO2 (black) films deposited on fused silica plates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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vs. hν plots to (αhν) = 0 depending on the structure of the semiconductor, where α and hν are the absorption coefficient and photon energy, respectively [33]. The square function is valid near a direct band gap hence the linear part of the former plot is used to determine direct gaps, while indirect band gaps usually meet square root relation (called the Tauc law) hence can be determined using the latter plot. The Tauc law is also valid for amorphous films [34]. Both the (αhν)2 vs. hν and (αhν)1/2 vs. hν plots have been used to determine the optical bands gap of HfO2 and ZrO2 films. No linear parts were found by Aarik et al. in the (αhν)2 vs. hν curve for their HfO2 thin films grown by atomic layer deposition at 500 K, by contrast, the (αhν)2 vs. hν curves of the films grown at 570–1200 K showed linear parts in a narrow energy range from which the films were estimated to have gap energies of 5.68– 5.72 eV [13]. From the (αhν)2 vs. hν plots for linear regression, Balog et al. determined the band gap energy Eg of 5.65 eV for their monoclinic HfO2 films grown by chemical vapor deposition [9]. Also using the same plot corresponding to the square function, Kosacki et al. estimated the band gap of ZrO2:Y films [10], whereas Kato et al. used the (αhν)1/2 vs. hν plot to estimate the band gap Eg of HfxSi1 − xOy and ZrxSi1 − x Oy [11]. For our HfO2 and ZrO2 films, it appears that the (αhν)2 vs. hν plot rather than the other one is suitable for band gap determination, as shown in Figs. 5 and 6, revealing the direct band and non-amorphous structure of the films. Linear dependence of the (αhν)2 on hν and its extrapolation to (αhν)2 = 0 gives the values of Eg = 5.65 eV and 5.96 eV for the PARPLD HfO2 and ZrO2 films, respectively. By comparing Fig. 5(a) with Fig. 6(a), however, it can be seen that the (αhν)2 vs. hν curve near band gap shows a good linearity for the ZrO2 films, while for the HfO2 films two distinct linear relations exist, suggesting the existence of two different absorption processes in the HfO2 films. The intercepts of the two lines with the photon energy axis (Eg1 and Eg2) represent the onsets for the two different absorption processes. Al-Kuhaili has also observed two linear relations in the (αhν)1/2 vs. hν curve for amorphous HfO2 films deposited by electron beam evaporation and attributed them to the intrinsic absorption of the oxide and the absorption involving some
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Fig. 6. Dependencies of (a) (αhν)2 on photon energy hν and (b) (αhν)1/2on photon energy hν, and determination of the band gap energy for ZrO2 films using (αhν)2 vs. hν plot.
defect states [4]. For our HfO2 films, the lower energy Eg1 = 5.65 eV is attributed to the intrinsic absorption of the oxide, however, at present we could not give an unambiguous explanation for the origin responsible for the other phonon energy Eg2 = 6.07 eV, a rather large energy value. From the (αhν)2 vs. hν plots presented in Fig. 5(a) and Fig. 6(a), in addition, one may speculate that the ZrO2 films have a better monoclinic structure than the HfO2 films, consistent with the observation obtained by the FTIR analysis for the HfO2 and ZrO2 films as described earlier. 4. Conclusions
Fig. 5. Dependencies of (a) (αhν)2 on photon energy hν and (b) (αhv)1/2on photon energy hν, and determination of the band gap energy for HfO2 films using (αhν)2 vs. hν plot.
Monoclinic HfO2 and ZrO2 thin films were depostited at low temperature from metallic hafnium or zirconium. The assistance of the reactive and energetic ECR oxygen plasma and the energetic gaseous precursors produced in the PARPLD process allowed the lowtemperature growth of oxides. The FTIR analysis of the deposited films revealed the characteristics of monoclinic phases of HfO2 and ZrO2, while no Si–O related modes were observed. The low-temperature growth and the concurrence of the ignition of oxygen plasma and the beginning of film deposition minimized the surface oxidation of Si substrates before and during film deposition, hence eliminated the possibility of the formation of interfacial SiOx layer between the oxide film and the Si substrate. The optical properties of the films on silicon substrates were studied by spectroscopic ellipsometry in the wavelength range 260–1700 nm and the optical constants were determined from the curve fitting of the ellipsometric data. The HfO2 and ZrO2 films deposited on fused silica plates show high transparency in the UV–Vis–NIR region with nearly constant transmittances above 85% in the NIR and sharp ultraviolet absorption edges near 200 nm. From the (αhν)2 vs. hν plots, the films were determined to have direct band gap energies of 5.65 eV for HfO2 and 5.96 eV for ZrO2, respectively.
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Acknowledgements The authors would like to acknowledge the support from the National Natural Science Foundation of China under the Contract No. 10875029. Shanghai AM Research and Development Foundation (No. 07SA11) and Shanghai Municipal Commission of Science and Technology (No. 08520706100) are also acknowledged. References [1] M.R. Kozlowski, in: F. Flory (Ed.), Thin Films for Optical Systems, Dekker, New York, 1996, p. 521. [2] C. Giuri, M.R. Perrone, V. Piccinno, Appl. Opt. 36 (1997) 1143. [3] J.D.T. Kruschwitz, W.T. Pawlewicz, Appl. Opt. 36 (1997) 2157. [4] M.F. Al-Kuhaili, Opt. Mater. 27 (2004) 383. [5] M. Zukic, D.G. Torr, J.F. Spann, M.R. Torr, Appl. Opt. 29 (1990) 4284. [6] S.M. Edlou, A. Smajkiewicz, G.A. Al-Jumaily, Appl. Opt. 32 (1993) 5601. [7] M. Gilo, N. Croitoru, Thin Solid Films 350 (1999) 203. [8] M. Alvisi, M. Di Giulio, S.G. Marrone, M.R. Perrone, M.L. Protopapa, A. Valentini, L. Vasanelli, Thin Solid Films 358 (2000) 250. [9] M. Balog, M. Schieber, M. Michman, S. Patai, Thin Solid Films 41 (1977) 247. [10] I. Kosacki, V. Petrovsky, H.U. Anderson, Appl. Phys. Lett. 74 (1999) 341. [11] H. Kato, T. Nango, T. Miyagawa, T. Katagiri, K.S. Seol, Y. Ohki, J. Appl. Phys. 92 (2002) 1106. [12] H. Hu, C. Zhu, Y.F. Lu, Y.H. Wu, T. Liew, M.F. Li, B.J. Cho, W.K. Choi, N. Yakovlev, J. Appl. Phys. 94 (2003) 551. [13] J. Aarik, H. Mändar, M. Kirm, L. Pung, Thin Solid Films 466 (2004) 41. [14] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530.
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