Monoxide Thin Films by Atomic Layer Deposition from Bis(1

Communication pubs.acs.org/cm

Growth of p‑Type Tin(II) Monoxide Thin Films by Atomic Layer Deposition from Bis(1-dimethylamino-2-methyl-2propoxy)tin and H2O Jeong Hwan Han,† Yoon Jang Chung,† Bo Keun Park,† Seong Keun Kim,‡ Hyo-Suk Kim,† Chang Gyoun Kim,*,† and Taek-Mo Chung*,† †

Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Republic of Korea ‡ Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul, 136-791, Republic of Korea S Supporting Information *

I

tetrakis(dimethylamino)tin/H2O (or H2O2), and (1,3-bis(1,1dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2ylidene)tin/NO (or H2O2)), all of these cases resulted in ntype SnO2 films.18−22 Thus, in this report, a stable and chemically reliable deposition method to achieve p-type SnO films is presented using ALD from the Sn(dmamp)2, bis(1-dimethylamino-2methyl-2propoxy)tin(II), and H2O. The Sn precursor was prepared by reacting the SnCl2 with lithium bis(trimethylsilyl)amide (Li(btsa)) in 50 mL of ether at room temperature for 3 h. The mixed solution was filtered for elimination of LiCl, and the remaining solvent was removed in vacuum conditions. The Sn(btsa)2 dissolved in hexane was mixed with dmampH at room temperature for 6 h. The final product was obtained as colorless liquid by distillation at 100 °C under 10−2 torr.23 The molecular structure and TGA/DTA characteristics of Sn precursor are provided in Supporting Information (Figure S1 and S2). For comparison, the ALD process using the same Sn precursor and O3 was conducted, which this time resulted in an n-type SnO2 film.24 The SnO ALD process was conducted at low temperatures ranging between 90 and 210 °C, which is compatible with conventional organic and flexible substrates. The Sn precursor was heated at 70 °C for sufficient vapor pressure and an Ar carrier gas flow of 100 sccm was introduced into the canister for source delivery. The temperature of the Sn precursor delivery line was maintained at 80−110 °C to prevent precursor condensation. 300 nm thick thermal SiO2 on Si was used as a substrate. Four sequential steps of (1) Sn precursor pulse, (2) Ar purge, (3) H2O pulse, and (4) Ar purge were applied for a single ALD cycle. To confirm the self-limiting growth characteristic of ALD, the variation in the area density of Sn (“the loading amount of Sn”) was measured by X-ray fluorescence (XRF) as a function of Sn precursor and H2O pulse lengths at the substrate temperature of 150 °C as shown in Figure 1a,b, respectively. Here, the purge times for Sn(dmamp)2 and H2O were fixed to 10 s and all samples were grown for 300 ALD cycles. As can be seen in the figures, it is clear that a Sn precursor pulse length of 5 s and H2O pulse length of 5 s are essential for proper ALD

n the past decade, oxide semiconductor materials have attracted great attention for emerging applications in thin film transistors (TFT), gas sensors, lithium batteries, and solar cells.1−3 A great deal of experimental study has been conducted on wide band gap n-type oxides such as ZnO, SnO2, ZnSnO, InGaZnO, and WO3, but p-type materials such as SnO, Cu2O, and N-doped ZnO are rather rarely explored due to the limitations in stable methods to achieve these systems.4−9 However, since many electronic applications implemented in the modern era require devices such as p−n junctions and complementary metal oxide semiconductor (CMOS) architectures, it is crucial that a reliable fabrication method for p-type oxide semiconductors be available for future technological advancement. Among the variety of p-type oxides, SnO, tin(II) monoxide, is of special interest because it has a wide optical band gap of 2.7−3.0 eV, which highlights the feasibility of completely transparent electronic devices, and moreover because SnO based TFT recently showed the record field effect mobility of ∼6.75 cm2/(V·s) and Hall mobility of ∼18.71 cm2/(V·s).10 To date, physical vapor deposition (PVD) methods such as sputtering, evaporation, and pulsed laser deposition were primary techniques to obtain p-type SnO.11−15 Furthermore, although these films could be demonstrated by PVD, the process window for single phase SnO is very narrow due to the metastable property of SnO.16,17 For this reason, during the preparation of SnO, it can easily transform into n-type SnO2 through decomposition reaction (2SnO → SnO2 + Sn) or oxidation (2SnO + O2 → 2SnO2). Meanwhile, for uniform and conformal thin film growth over complex and three-dimesional substrates, it is required to use chemical vapor deposition (CVD) or atomic layer deposition (ALD). Since many modern device apertures are designed to have topographically diverse structures to maximize performance, these chemistry based methods should also be thoroughly investigated for means to fabricate SnO films. Suh et al. demonstrated the CVD of SnO film using Sn(II) precursor, Sn(OCH(CF3)2)2(HNMe2), in combination with H2O, and showed that the formation of SnO or SnO2 can be realized selectively by the precursor based oxidation state control.7 In the case of ALD, however, although a wide variety of Snprecursor/reactant combinations have been explored to grow SnOx (x = 1 or 2) films (e.g., SnCl4/H2O, SnI4/H2O, © 2014 American Chemical Society

Received: August 24, 2014 Revised: October 8, 2014 Published: October 21, 2014 6088

dx.doi.org/10.1021/cm503112v | Chem. Mater. 2014, 26, 6088−6091

Chemistry of Materials

Communication

mechanism of NiO ALD using Ni(dmamp)2 and H2O, where the release of dmampH was clearly observed.26 The impact of the growth temperature on the film density is investigated by X-ray reflectivity (XRR) as shown in Figure 1e. From the critical angle, it is believed that the film density is abruptly changed at temperatures between 120 and 150 °C. SnO films with lower density are obtained at 90 and 120 °C. By contrast, denser films are grown at temperatures higher than 150 °C. The density of the as-deposited films grown at 90−120 °C is 4.3−4.6 g/cm3, while that of the films grown at the higher temperatures of 150−210 °C is ∼5.5 g/cm3. The notable change in density might be ascribed to the crystallization of the SnO film when the growth temperature is over 150 °C (see Figure 2a and Supporting Information Figure 6S). The

Figure 1. Variations of the area density of Sn as a function of (a) Sn(dmamp)2 and (b) H2O pulse length. (c) Change in film thickness as a function of the number of ALD cycles. (d) Changes in the area density of Sn and GPC as a function of the deposition temperature. (e) Measured XRR profiles (symbol) and simulated curves (line) for SnO films grown at 90−180 °C. Figure 2. (a) XRD pattern for the SnO films grown on SiO2 for 700 ALD cycles at 90−180 °C. (b) Cross-sectional HRTEM image of SnO film deposited at 150 °C. Inset shows FFT pattern of SnO crystalline.

growth. After establishing surface saturation conditions, which correspond to the sequential pulse steps of 5 s−10 s−5 s−10 s, SnO ALD films were deposited as a function of ALD cycle number at 150 °C (Figure 1c). The variation of film thickness according to the number of deposition cycles showed a clear linear profile, which indicates a growth characteristic unique to ALD. The growth per cycle (GPC) achieved for this condition was 0.18 Å/cycle. Step coverage characteristic of the ALD grown SnO film was also confirmed on a hole pattern with an opening diameter of 60 nm and depth of 1500 nm (aspect ratio of 25) as shown in Supporting Information Figure S3. SnO films could be conformally deposited inside the hole structure, which corroborates the self-limiting growth characteristic of the SnO ALD process proposed in this work. Figure 1d depicts the changes in the Sn layer density and GPC of the deposited films at various growth temperatures ranging from 90 to 210 °C. The GPC and Sn layer density are decreasing with increasing growth temperature. This tendency is in accordance with the previous reports on ALD oxide processes using H2O as a reactant, where the OH density on the reaction surface governs the chemisorption rate of the precursor and hence the GPC.25 Accordingly, it can be presumed that the Sn(dmamp)2/H2O reaction chemistry strongly relies on a hydroxylation-mediated reaction as proposed in the following:

obtained film densities at 150−180 °C are lower than the theoretical density of a stoichiometric SnO film (6.45 g/cm3). The AFM and SEM images in Supporting Information Figures S4 and S5 reconfirm this tendency as can be seen from the evolution of surface morphology according to growth temperature. RMS roughnesses of 20−25 nm-thick SnO films grown at 90−120 °C and 150−210 °C are 0.19−0.25 and 3.2−5.1 nm, respectively. To identify the crystallinity and phase of the resultant films grown at 90−180 °C, namely, to verify whether SnO or SnO2 was deposited, X-ray diffraction (XRD) was performed in θ−2θ mode as illustrated in Figure 2a. At 90 and 120 °C, the XRD patterns show no observable peak, which indicates formation of an amorphous film most likely due to the relatively low growth temperatures. On the other hand, at the higher temperatures of 150 and 180 °C, the as-deposited films exhibited clear crystalline peaks corresponding to tetragonal α-SnO (001) and (002) peaks at 18.3° and 37.1°, respectively. Although an α-SnO (102) distinctive peak was also found from glancing angle XRD measurements (Supporting Information Figure 6S), it is clear that the SnO ALD film grown at 150 °C has a highly (001) textured structure. Interestingly, any undesired phases in the SnO matrix such as SnO2 or metallic Sn were not found from the (GA)XRD results, which suggests the formation of a single-phase SnO film. To further verify this observation, high resolution transmission electron microscopy (HRTEM) was used to examine the microstructure of SnO film (20 nm) grown on SiO2 at 150 °C. The cross-sectional HRTEM image in Figure 2b shows that the as-deposited SnO film has readily apparent crystallinity with a d-spacing of 0.26 nm, which directly corresponds to the planar distance of the (110) plane in SnO.27 The fast Fourier transform (FFT) pattern achieved for the same sample revealed

OH* + Sn(dmamp)2 (g) → O−Sn(dmamp)* + dmampH(g)

(1)

O−Sn(dmamp)* + H 2O(g) → O−Sn−OH* + dmampH(g)

(2)

where * represents the surface bounded species after each halfcycle. Although in situ analysis on the reaction mechanism was not covered in this study, the suggested mechanism is plausible when considering the author’s previous report on the reaction 6089

dx.doi.org/10.1021/cm503112v | Chem. Mater. 2014, 26, 6088−6091

Chemistry of Materials

Communication

deposited films. Figure 3c depicts the O/Sn atomic ratios of SnO films grown at various temperatures along with the case of the O3−SnO2 film. O/Sn ratios of approximately 1.2 were achieved for all samples, independent of deposition temperature. In contrast to the SnO film grown from H2O, the O3− SnO2 film showed O/Sn ratio of 1.8. This discrepancy in atomic ratio can be understood from the difference in the resultant film phase depending on oxidant type used and is in good agreement with the XPS binding energy results. Rutherford backscattering spectroscopy (RBS) was conducted for further accurate composition analysis of the SnO film grown at 150 °C. It was revealed that a SnO1.2 film was deposited, which is very well matched with the XPS results. From the oxygen rich composition of the p-type SnO, it is speculated that the achieved SnO film contains either Sn vacancies and/or O interstitials of which defect formation are energetically favorable compared to Sn interstitial and O vacancy.29 Moreover, it was found that the ALD SnO film features uniform atomic distribution in the depth direction as well as negligible C and N concentrations below the detection limit from the auger depth profile (AES) in the inset of Figure 3d. The optical properties of the p-type SnO films were also examined to determine the band gap and transmittance by ultraviolet−visible spectroscopy (UV−vis) as shown in Figures 4a,b. The UV−vis result for the O3−SnO2 film was also

a textured structure along the [110] direction, which is consistent with the HRTEM image. The chemical composition of the deposited films was also investigated to identify their stoichiometry. Figure 3a shows the

Figure 3. (a) XP spectra of Sn 3d core level of SnO films deposited at 90−180 °C (after 20 s Ar etching). For comparison, Sn 3d spectrum for O3−SnO2 grown at 150 °C was included. (b) XP spectra of C 1s and N 1s core level of SnO film deposited at 150 °C before and after surface etching. (c) O/Sn atomic ratio of SnO films grown at 90−180 °C. O/Sn ratio for O3−SnO2 film was presented for comparison. (d) RBS spectrum of SnO film grown at 150 °C. Inset shows a representative AES depth profile of SnO film grown at 150 °C on SiO2 substrate. Figure 4. (a) Change in absorbance of SnO films and O3−SnO2 film over the wavelength range of 200−800 nm. (b) Transmittance of SnO and O3−SnO2 films deposited on quartz substrate.

X-ray photoelectron (XP) spectra of the Sn 3d core level for SnO films grown at various growth temperatures of 90, 120, 150, and 180 °C. For a clear comparison in binding state energies, the XP spectrum for an n-type SnO2 film grown with the same Sn precursor and O3 (denoted as O3−SnO2) at 150 °C is also given in the figure. According to the author’s exploratory experiments, it was found that ALD using Sn(dmamp)2 with O3 or O2 plasma leads to the formation of n-type SnO2 owing to the high oxidation power of these reactants.23,28 It should be noted that, for the SnO films, all the Sn 3d core level spectra could be fitted as a single peak with a binding energy of approximately 485.9 eV, irrespective of deposition temperature. As the Sn 3d binding energy for the SnO ALD film is much closer to that of reported values for stoichiometric SnO (∼486.2 eV) rather than for stoichiometric SnO2 (∼486.6 eV), it is believed that single phase SnO films with a Sn oxidation state of +2 were deposited. By contrast, the Sn 3d core level spectrum for the O3−SnO2 film was shifted toward a higher binding energy of 486.9 eV because the Sn oxidation state is more likely closer to +4. Figure 3b shows the C 1s and N 1s core level spectra of the SnO film deposited at 150 °C. The C and N concentrations are below the detection limit, indicating that an impurity free p-type SnO film was grown. At all investigated temperatures, SnO films were grown without C and N contamination. Additionally, quantitative analysis on the O/Sn atomic ratios was carried out for all the

included for comparison. The p-type SnO films at the temperatures of 120−180 °C show a band gap of 2.6−2.7 eV with the presence of a sharp absorption at band-edge. These values are in agreement with previous reports on the optical band gap of p-type SnO.17 In comparison, the n-type SnO2 exhibited an optical band gap of 3.6 eV, and a highly transparent (>90%) film was achieved in the wavelength range of 200−800 nm. Although the SnO film also exhibits high transparency in the range of 500−800 nm, it rapidly decreases at 460 nm which accurately corresponds with the achieved optical band gap. Finally, the electrical characteristics of the ALD SnO films were investigated. From the previous paragraphs, it can be inferred that holes should be the majority carriers in the films. Figure 5 shows the carrier concentration and Hall mobility of the SnO films grown at 150−210 °C obtained by Hall measurements. The amorphous SnO films deposited at 90 and 120 °C did not provide reliable data due to their low carrier concentration. From the positive Hall coefficient, it could be confirmed that the SnO films grown at 150−210 °C portray a p-type nature with carrier concentrations of 3.4 × 1017 to 1.6 × 1018 cm−3. Hall mobility and resistivity of ALD SnO films were 6090

dx.doi.org/10.1021/cm503112v | Chem. Mater. 2014, 26, 6088−6091

Chemistry of Materials

Communication

(5) Park, S.-H. K.; Hwang, C.-S.; Jeong, H. Y.; Chu, H. Y.; Cho, K. I. Electrochem. Solid-State Lett. 2008, 11, H10. (6) Sung, S.-Y.; Kim, S.-Y.; Jo, K.-M.; Lee, J.-H.; Kim, J.-J.; Kim, S.-G.; Chai, K.-H.; Pearton, S. J.; Norton, D. P.; Heo, Y.-W. Appl. Phys. Lett. 2010, 97, 222109. (7) Suh, S.; Hoffman, D. M.; Atagi, L. M.; Smith, D. C.; Liu, J.-R.; Chu, W.-K. Chem. Mater. 1997, 9, 730. (8) Liu, W.; Xiu, F.; Sun, K.; Xie, Y.-H.; Wang, K. L.; Wang, Y.; Zou, J.; Yang, Z.; Liu, J. J. Am. Chem. Soc. 2010, 132, 2498. (9) Chavillon, B.; Cario, L.; Renaud, A.; Tessier, F.; Cheviré, F.; Boujtita, M.; Pellegrin, Y.; Blart, E.; Smeigh, A.; Hammarström, L.; Odobel, F.; Jobic, S. J. Am. Chem. Soc. 2012, 134, 464. (10) Caraveo-Frescas, J. A.; Nayak, P. K.; Al-Jawhari, H. A.; Granato, D. B.; Schwingenschlögl, U.; Alshareef, H. N. ACS Nano 2013, 7, 5160. (11) Fortunato, E.; Barros, R.; Barquinha, P.; Figueiredo, V.; Park, S.H. K.; Hwang, C.-S.; Martins, R. Appl. Phys. Lett. 2010, 97, 052105. (12) Dhananjay, C. W.; Chu, C. W.; Ou, M. C.; Wu, Z. Y.; Ho, K. C.; Lee, S. W. Appl. Phys. Lett. 2008, 92, 232103. (13) Yabuta, H.; Kaji, N.; Hayashi, R.; Kumomi, H.; Nomura, K.; Kamiya, T.; Hirano, M.; Hosono, H. Appl. Phys. Lett. 2010, 97, 72111. (14) Nomura, K.; Kamiya, T.; Hosono, H. Adv. Mater. 2011, 23, 3431. (15) Ogo, Y.; Hiramatsu, H.; Nomura, K.; Yanagi, H.; Kamiya, T.; Hirano, M.; Hosono, H. Appl. Phys. Lett. 2008, 93, 032113. (16) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Adv. Funct. Mater. 2003, 13, 9. (17) Pan, X. Q.; Fu, L. Appl. Phys. Lett. 2001, 89, 6048. (18) Du, X.; Du, Y.; George, S. M. J. Vac. Sci. Technol. A 2005, 23, 581. (19) Sundqvist, J.; Tarre, A.; Rosental, A.; Hårsta, A. Chem. Vap. Deposition 2003, 9, 21. (20) Elam, J. W.; Baker, D. A.; Hryn, A. J.; Martinson, A. B. F.; Pellin, M. J.; Hupp, J. T. J. Vac. Sci. Technol. A 2008, 26, 244. (21) Heo, J.; Hock, A. S.; Gordon, R. G. Chem. Mater. 2010, 22, 4964. (22) Mullings, M. N.; Hägglund, C.; Bent, S. F. J. Vac. Sci. Technol. A 2013, 31, 061503. (23) Kim, C. G.; Chung, T.-M.; Lee, Y. K.; An, K.-S.; Lee, S. S.; Ryu, B. H.; Jang, S. J. U.S. Patent 8030507, 2011. (24) Choi, M.-J.; Cho, C. J.; Kim, K.-C.; Pyeon, J. J.; Park, H.-H.; Kim, H.-S.; Han, J. H.; Kim, C. G.; Chung, T.-M.; Park, T. J.; Kwon, B.; Jeong, D. S.; Baek, S.-H.; Kang, C.-Y.; Kim, J.-S.; Kim, S. K. Appl. Surf. Sci. 2014, 320, 188. (25) Nyns, L.; Delabie, A.; Caymax, M.; Heyns, M. M.; Van Elshocht, S.; Vinckier, C.; De Gendta, S. J. Electrochem. Soc. 2008, 155, G269. (26) Yang, T. S.; Cho, W.; Kim, M.; An, K.-S.; Chung, T.-M.; Kim, C. G.; Kim, Y. J. Vac. Sci. Technol. A 2005, 23, 1238. (27) Caraveo-Frescas, J. A.; Alshareef, H. N. Appl. Phys. Lett. 2013, 103, 222103. (28) Lee, B. K.; Jung, E.; Kim, S. H.; Moon, D. C.; Lee, S. S.; Park, B. K.; Hwang, J. H.; Chung, T.-M.; Kim, C. G.; An, K.-S. Mater. Res. Bull. 2012, 47, 3052. (29) Togo, A.; Oba, F.; Tanaka, I.; Tatsumi, K. Phys. Rev. B 2006, 74, 195128. (30) Guo, W.; Fu, L.; Zhang, K.; Liang, L. Y.; Liu, Z. M.; Cao, H. T.; Pan, X. Q. Appl. Phys. Lett. 2010, 96, 042113.

Figure 5. (a) Variation of carrier concentration and Hall mobility of as-deposited SnO films grown at 150−210 °C and (b) changes in the carrier concentration and Hall mobility of as-deposited SnO films grown at 150 °C as a function of time after air exposure.

found to be 0.4−2.9 cm 2/(V·s) and 4.9−14.5 Ω·cm, respectively, which are comparable with those of PVD SnO films.30 In summary, we successfully developed an ALD process for single-phase SnO films with negligible impurity levels at low deposition temperatures of 90−210 °C from the bis(1dimethyamino-2-methyl-2propoxy)tin(II) precursor and H2O. The SnO ALD exhibited self-limiting growth behavior with excellent conformality. The growth rates of the p-type SnO film were varied from 0.61 to 0.08 Å/cycle with increasing the growth temperature from 90 to 210 °C. XRD and TEM images revealed that (001) textured crystalline SnO was deposited at growth temperatures over 150 °C, whereas amorphous SnO film was grown below 120 °C. SnO ALD films showed p-type conductivity under a wide range of deposition conditions and quite reasonable Hall mobility values of 0.4−2.9 cm2/(V·s).

■

ASSOCIATED CONTENT

S Supporting Information *

Thermogravimetric analysis (TGA)/differential thermal analysis (DTA), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and glancing angle X-ray diffraction (GAXRD). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*(C.G.K.) E-mail: [email protected]. *(T.-M.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to acknowledge the financial support from the R&D Convergence Program of NST (National Research Council of Science & Technology) of Republic of Korea.

■

REFERENCES

(1) Fortunato, E.; Barquinha, P.; Martins, R. Adv. Mater. 2012, 24, 2945. (2) Kim, H.-J.; Lee, J.-H. Sens. Actuators, B 2014, 192, 607. (3) Jose, R.; Thavasi, V.; Ramakrishna, S. J. Am. Ceram. Soc. 2009, 92, 289. (4) Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H. Nature 2004, 432, 488. 6091

dx.doi.org/10.1021/cm503112v | Chem. Mater. 2014, 26, 6088−6091