- Email: [email protected]
How morphology determines the charge storage properties of Ge nanocrystals in HfO2
Scripta Materialia 113 (2016) 135–138
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
How morphology determines the charge storage properties of Ge nanocrystals in HfO2 A. Slav a, C. Palade a,b, A.M. Lepadatu a,⁎, M.L. Ciurea a,c, V.S. Teodorescu a, S. Lazanu a, A.V. Maraloiu a, C. Logofatu a, M. Braic d, A. Kiss d a
National Institute of Materials Physics, 105 bis Atomistilor Street, 077125 Magurele, Romania University of Bucharest, Faculty of Physics, 405 Atomistilor Street, 077125 Magurele, Romania Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucuresti, Romania d National Institute for Optoelectronics, 409 Atomistilor Street, 077125 Magurele, Romania b c
a r t i c l e
i n f o
Article history: Received 1 October 2015 Accepted 13 October 2015 Available online xxxx Keywords: Nanostructured materials High-angle annular dark field (HAADF) High-resolution electron microscopy (HREM) X-ray photoelectron spectroscopy (XPS) Charge storage properties
a b s t r a c t The strong correlation between morphology and charge storage properties of HfO2/Ge/HfO2/Si trilayer structures was evidenced. The morphology of structures deposited by magnetron sputtering and electron beam evaporation was tailored by rapid thermal annealing and investigated by transmission electron microscopy, Raman and X-ray photoelectron spectroscopies. The best hysteresis loops (capacitance–voltage characteristics) were obtained for trilayers with high density Ge nanocrystals located in the position of as-deposited Ge layer. The decrease of Ge nanocrystals density narrows the memory window, by spreading Ge atoms into HfO2 matrix (sputtered trilayers), or by Ge atoms expulsion toward HfO2 nanocrystals surface (evaporated trilayers). © 2015 Elsevier Ltd. All rights reserved.
The memory properties of Si and Ge nanocrystals (NCs) embedded in various insulator matrices, including high-k oxides have been intensively studied for their applications in nonvolatile memories (NVMs) [1–10]. By using Ge NCs instead of Si NCs, the stronger quantum confinement effect [7] is exploited for improving the memory properties. Downscaling of device size conduced to the replacement of SiO2 as tunnel and/or gate oxides in MOS-like devices with a high-k dielectric such as HfO2 [11]. The most used deposition methods for obtaining MOS-like NVMs based on Ge NCs are magnetron sputtering (MS), electron beam evaporation (EBE), chemical vapour deposition and molecular beam epitaxy [8,11–13]. The size and density [14–15] of Ge NCs in the intermediate Ge layer positioned between tunnel and gate oxides together with the location [16] of intermediate layer are well controlled by using rapid thermal annealing (RTA). Also, RTA processing may induce traps at the Ge NCs/matrix interface and/or at oxide/Si interface [17–18]. Although the charge storage properties of structures with Ge NCs embedded in HfO2 matrix, prepared under different conditions were extensively investigated, their correlation with morphology was not systematically studied. So, in this paper the relationship between morphology and memory properties of structures based on Ge NCs embedded in HfO2 matrix is investigated. For this purpose, HfO2/Ge/HfO2/Si trilayer structures were deposited by using either MS or EBE methods, ⁎ Corresponding author. E-mail address: lepadatu@infim.ro (A.M. Lepadatu).
http://dx.doi.org/10.1016/j.scriptamat.2015.10.028 1359-6462/© 2015 Elsevier Ltd. All rights reserved.
and their morphology was tailored by subsequent RTA under different conditions. The morphology of trilayers was studied by using highresolution transmission electron microscopy (HRTEM), high-angle annular dark field-scanning TEM (HAADF-STEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The charge storage properties were investigated by measuring capacitance–voltage (C–V) at different frequencies and capacitance–time (C–t) characteristics. We show that the only contributors to the charge storage properties of our trilayer structures are Ge NCs with high density located in the position of as-deposited Ge layer. We prepared trilayer structures (MOS-like capacitors) with Ge NCs embedded in HfO2 on p-type Si (100) wafers with 7–14 Ωcm resistivity. Si wafers were classically cleaned in wet chemical solution, followed by dipping in diluted HF solution (2%) for removing the native Si oxide. The trilayers were deposited by MS in pure Ar (6N) of 4 mTorr pressure, by using 40–60 W RF for HfO2 and 5–10 W DC for Ge. So, the tunnel HfO2 layer has 5–10 nm thickness, the intermediate Ge layer has 5–10 nm and the gate HfO2 layer 20–50 nm. For EBE deposition, layers with similar thicknesses were prepared by using a rate of about 0.025 nm/s for HfO2 and 0.054 nm/s for Ge. We also prepared control capacitors (MOS capacitors) without Ge content using similar conditions to the trilayer capacitors. All MS and EBE structures (including control ones) were deposited on Si wafers kept at room temperature. All as-deposited trilayers are amorphous, therefore for Ge nanostructuring they were annealed in a rapid thermal processor in N2 atmosphere (6N), at different temperatures between 600 and 1000
136
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
°C for 3–9 min. The as-deposited control structures are also amorphous and after RTA performed similarly to the trilayer structures, HfO2 becomes crystallized [19]. The MOS-like capacitors were obtained by thermal evaporation of Al contacts with area of 6.4 × 10−3 cm2 on the top and backside of the trilayer structures. The morphology of trilayer structures was investigated by using a Jeol ARM 200F electron microscope, a confocal T64000 Horiba Jobin Yvon Raman spectrometer (514 nm Ar laser) and a SPECS XPS spectrometer (Phoibos 150 MCD electron energy analyser, constant energy mode, monochromatic X-ray radiation Al Kα 1486.74 eV). Fig. 1(a, b and c) presents HRTEM images of MS trilayers showing the cross-section morphology evolution from the as-deposited structures to structures annealed at 600 and 850 °C for 8 min (MS-600 and MS-850, respectively). As one can see, the as-deposited trilayer structure is amorphous. MS-600 sample is crystallized and keeps the trilayer morphology, the intermediate Ge layer being visible. For MS-850 structure, the trilayer morphology is less visible and HfO2 is crystallized all over the structure thickness. At the interface of deposited layers with the Si substrate an amorphous SiO2 layer grown during deposition [20–21] is visible, that is thicker in MS-850 sample than in MS-600. The HAADF-STEM and HRTEM images of the cross-section of MS600 structures are compared in Fig. 1(d and e). The Z contrast in the HAADF image reveals the presence of a row of Ge nanoparticles (NPs) with sizes of 5–7 nm, located in the initial position of as-deposited Ge layer. The Z contrast shows only the agglomeration of Ge atoms (in NPs) that can be or not in an oxidation state. The Ge NCs embedded in crystalline HfO2 cannot be evidenced in TEM [22], due to their similar lattice constants. In Fig. 1(f), the Raman spectrum measured on MS-600 together with the fit curve using Richter phonon quantum confinement model [16] are presented. The peak located at 299.2 cm−1 corresponds to Ge NCs with 6.5 nm average size, showing that the Ge NPs evidenced in microscopy are crystallized. Fig. 1(g) shows XPS spectra of Ge 2p3/2 measured on MS-600 trilayers at different depths under the free surface after trilayer sputtering (Ar+) with about 0.5 nm/min rate. In the curve taken at the free surface, the peak located at 1220.6 eV corresponds to fully oxidized Ge (GeO2) [23]. The first two sputtering steps (1 and 2 at about 2.5 and 5 nm depths, respectively) do not evidence any Ge signal. Then, the curves obtained after steps 3, 4 and 5 (22.5, 25 and 27.5 nm depths) are formed of two bands, one band corresponds to metallic Ge positioned at 1217 eV [23] and the other one can be attributed to partially oxidized Ge [24–25]. In Refs. [26–27], metallic Ge and partially oxidized Ge were evidenced in Ge 3d band. In Fig. 1(g), for steps 3, 4 and 5, the ratios of the intensities of metallic and oxidized Ge peaks are 42/58, 60/40 and 44/56, respectively. In the curve obtained after step 4 (at depth corresponding to intermediate layer of Ge NCs) the metallic Ge contribution overcomes the contribution of oxidized Ge (60/40). The decrease of intensities ratio after step 5 (44/56) shows the decrease of metallic Ge amount with respect to oxidized Ge as the sputtering depth is close to the tunnelling HfO2 layer depth. In Fig. 2, HRTEM and HAADF-STEM images taken on EBE-850 and EBE-900 structures annealed for 4 min are shown. In EBE-850 structures (Fig. 2(a and b)), Ge NPs of 5–6 nm size are located in the position of asdeposited Ge layer, similarly to MS-600 structure. Ge NPs are also crystallized as Raman measurements reveal. In EBE-900 (Fig. 2(c and d)), the trilayer morphology is lost and the sample contains HfO2 NCs that are extended over all the trilayer thickness (HRTEM image). Ge NPs with sizes of 5–6 nm are visible in the bottom part of the HfO2 NCs layer and in between the HfO2 NCs (HAADF image). The growth process of HfO2 NCs corresponding to 900 °C annealing eliminates Ge atoms from the HfO2 lattice toward HfO2 NCs surface. Therefore, the Ge atoms segregate either at the bottom part of HfO2 NCs i.e. on the top of SiO2 layer or in between the HfO2 NCs, while Ge atoms arriving at the free surface are either oxidized or lost.
Fig. 1. HRTEM images of MS HfO2/Ge/HfO2/Si trilayers — cross-section morphology evolution from (a) as-deposited to (b) MS-600 and (c) MS-850 structures; (d)–(g) measurements on MS-600: (d) HAADF-STEM and (e) HRTEM cross-sectional images; (f) experimental and fit Raman spectra and (g) XPS spectra and deconvolution curves at the free surface and after different sputtering steps (1, 2, 3, 4 and 5).
The memory characteristics of MOS-like capacitors based on the trilayer structures with the morphologies described above are presented in the following. C–V characteristics were measured in the dark at room temperature (Agilent E4980A LCR Meter).
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
137
Fig. 2. HRTEM and HAADF-STEM images of the same area of EBE-850 (a and b) and EBE-900 structures (c and d).
In Fig. 3(a), the C–V characteristics recorded on the capacitors based on MS-600 trilayers (MS-600 capacitors) in the voltage range of (−4,+4) V, and at frequencies of 1 MHz, 500 and 100 kHz are given. The C–V characteristics show a hysteresis with a memory window ΔV of about 1 V that is frequency-independent meaning that the hysteresis loop is due to Ge NCs [18]. The inset contains the charge retention curve (C–t), taken on the MS-600 capacitor. For this, firstly the capacitor was charged for 3 min at − 4 V, then the voltage was changed to the flatband voltage, and then the C–t curve was recorded. After 4000 s, the capacitance decreased with 28%. In Fig. 3(b), C–V characteristics measured at 1 MHz, on MS-600 and MS-850 capacitors and also on the HfO2-600 control capacitor (without Ge) are shown. One can see that the control capacitor exhibits no memory window although in literature, important memory effects were reported on HfO2 charge trapping flash memories [28–30]. Therefore, in our structures, the traps present in HfO2 do not contribute to charge storage. The increase of annealing temperature from 600 to 850 °C produces the decrease of ΔV to 0.2 V (MS-850 capacitors), and the appearance of a small hump at positive voltages (inset) that is the signature of slow traps located at the Si/SiO2 interface [31]. Similarly to MS-600, ΔV
is independent of frequency. The MS capacitors annealed below 600 °C present narrower hysteresis loops. C–V measurements were also performed on EBE capacitors in the (−4,+4) V voltage interval at frequencies of 1 MHz, 600 and 100 kHz. The C–V curves obtained on the EBE-850 capacitor are shown in Fig. 3(c). They have a memory window ΔV of 0.8 V that is also frequency-independent as for MS-600 capacitors, the hysteresis being also due to Ge NCs. The C–V characteristics taken on control capacitors are similar to MS case, namely they exhibit no memory window. Additionally, the hysteresis loops shift toward more negative voltages when the frequency increases, which points to the presence of fast traps possibly located at the interface between the Ge NCs and HfO2 matrix [18]. This frequency-dependent shift has not been observed in MS capacitors (Fig. 3(a)). All hysteresis curves are centred on negative voltages (Fig. 3(c)). By increasing the annealing temperature from 850 to 1000 °C, they additionally shift toward more negative voltages and ΔV decreases, as can be seen in Fig. 3(d). Also, ΔV is independent of frequency for EBE-900 and EBE-1000 capacitors. For RTA below 850 °C, narrower memory windows were obtained. The C–t curve taken on EBE-850 is also represented in the inset, showing a decrease of 25% after 4000 s.
Fig. 3. (a) C–V characteristics of MS-600 capacitors measured at different frequencies; inset — C–t characteristic; (b) C–V characteristics of MS-600, MS-850 and control (HfO2-600) capacitors; inset — detail of MS-850 curve. (c) C–V characteristics of EBE-850 capacitors measured at different frequencies; inset — C–t and (d) C–V characteristics of EBE-850, EBE-900 and EBE-1000 capacitors.
138
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
The best hysteresis loops were obtained on the MS-600 (8 min RTA) and EBE-850 (4 min RTA) capacitors. In our opinion, they are due only to the Ge NCs embedded in HfO2 as it results from the electrical behaviour of capacitors, i.e. frequency-independence of memory windows [18,32] corroborated with no memory effect in capacitors without Ge content. This is also supported by XPS (Ge located in the position of asdeposited layer is in metallic form) and by Raman spectrum (Ge NCs of 6.5 nm size) for MS samples. Therefore, the Ge NPs observed in HRTEM and HAADF-STEM images are crystallized (Fig. 1(b, d and e) for MS-600 and Fig. 2(a and b) for EBE-850). In the case of MS structures, the increase of the annealing temperature produces Ge spreading into the crystallized HfO2 matrix (Fig. 1(c)). The formation of Ge NCs is hindered by Ge fast diffusion in the HfO2 lattice [16] that leads to the decrease of Ge NCs density and therefore to the narrowing of the memory window (Fig. 3(b)). In EBE structures, the increase of the annealing temperature leads to the segregation of Ge atoms at the surface of HfO2 NCs (at the HfO2/SiO2 interface or at the interface between HfO2 NCs — Fig. 2(c and d)). In other words, the crystal growth process of HfO2 NCs during annealing at 900 °C eliminates the Ge atoms from the HfO2 lattice toward HfO2 NCs surface, and thus the density of Ge NCs located inside HfO2 NCs diminishes, and therefore ΔV decreases. It results that the best memory characteristics found in MS-600 and EBE-850 capacitors are due to the high density of Ge NCs embedded in HfO2 matrix, located in the initial position of Ge layer. By tailoring the morphology of trilayers through the increase of annealing temperature, the hysteresis loops are narrowed as a result of Ge atoms spreading into the HfO2 matrix (in MS-850) or of their segregation at HfO2 NCs surface (in EBE-900). In summary, we have shown the determinant influence of morphology on charge storage properties of HfO2/Ge/HfO2/Si trilayer structures. So, by tailoring the morphology, the memory properties of these trilayers are tuned. The investigated structures were deposited by MS and EBE methods and nanostructured by RTA under different conditions. We have found that the best hysteresis loops are obtained on capacitors with similar morphologies, namely on MS-600 (8 min RTA) and EBE-850 (4 min RTA). In these trilayer structures, Ge NCs embedded in crystallized HfO2 are located in the position of as-deposited Ge layer, have a high density and are the only contributors to the charge storage properties. By changing the morphology of trilayers with the increase of annealing temperature, the density of Ge NCs is decreased producing a narrowing of the memory windows. So, in MS trilayer structures the Ge NCs formation is hindered due to the spreading of Ge atoms into the HfO2 matrix, while in EBE structures Ge atoms are expulsed from the HfO2 lattice during crystal growth process toward HfO2 NCs surface. The understanding of the relationship between morphology and charge storage properties is useful for the preparation of materials with improved properties for NVMs applications. Acknowledgements The authors would like to thank Prof. Ioan Baltog for the helpful discussions related to Raman results. This work was supported by the Romanian National Authority for Scientific Research through the
CNCS-UEFISCDI Contracts No. PN II-PT-PCCA-9/2012 and No. PNII-ID/ 289-2011. References [1] T.-C. Chang, F.-Y. Jian, S.-C. Chen, Y.-T. Tsai, Mater. Today 14 (2011) 608–615. [2] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbe, K. Chan, Appl. Phys. Lett. 68 (1996) 1377–1379. [3] G. Gay, G. Molas, M. Bocquet, E. Jalaguier, M. Gély, L. Masarotto, J.P. Colonna, H. Grampeix, F. Martin, P. Brianceau, V. Vidal, R. Kies, T. Baron, G. Ghibaudo, B. De Salvo, IEEE Trans. Electron Devices 59 (2012) 933–940. [4] J.S. de Sousa, R. Peibst, M. Erenburg, E. Bugiel, G.A. Farias, J.P. Leburton, K.R. Hofmann, IEEE Trans. Electron Devices 58 (2011) 376–383. [5] J. Martín-Sánchez, A. Chahboun, M.J.M. Gomes, A.G. Rolo, B. Pivac, I. Capan, Phys. Status Solidi (RRL) 6 (2012) 223–225. [6] R. Gupta, W.J. Yoo, Y. Wang, Z. Tan, G. Samudra, S. Lee, D.S.H. Chan, K.P. Loh, L.K. Bera, N. Balasubramanian, D.-L. Kwong, Appl. Phys. Lett. 84 (2004) 4331–4433. [7] S.K. Ray, S. Maikap, W. Banerjee, S. Das, J. Phys. D: Appl. Phys. 46 (2013) 153001. [8] S. Wang, W. Lu, Q. Wan, J.Y. Dai, P.F. Lee, L. Suhua, Q. Shen, M. Zhang, Z. Song, C. Lin, Appl. Phys. Lett. 86 (2005) 113105. [9] B.S. Sahu, F. Gloux, A. Slaoui, M. Carrada, D. Muller, J. Groenen, C. Bonafos, S. Lhostis, Nanoscale Res. Lett. 6 (2011) 177. [10] D. Lehninger, P. Seidel, M. Geyer, F. Schneider, V. Klemm, D. Rafaja, J. von Borany, J. Heitmann, Appl. Phys. Lett. 106 (2015) 023116. [11] S. Das, K. Das, R.K. Singha, A. Dhar, S.K. Ray, Appl. Phys. Lett. 91 (2007) 233118. [12] Y.Q. Wang, J.H. Chen, W.J. Yoo, Y.-C. Yeo, Mater. Res. Soc. Symp. Proc. 830 (2005) 269–274. [13] R. Aluguri, S. Das, R.K. Singha, S.K. Ray, Curr. Appl. Phys. 13 (2013) 12–17. [14] L.W. Teo, W.K. Choi, W.K. Chim, V. Ho, C.M. Moey, M.S. Tay, C.L. Heng, Y. Lei, D.A. Antoniadis, E.A. Fitzgerald, Appl. Phys. Lett. 81 (2002) 3639–3941. [15] C.G. Ahn, H.S. Kang, Y.K. Kwon, S.M. Lee, B.R. Ryum, B.K. Kang, J. Appl. Phys. 86 (1999) 1542–1547. [16] A.-M. Lepadatu, T. Stoica, I. Stavarache, V.S. Teodorescu, D. Buca, M.L. Ciurea, J. Nanopart. Res. 15 (2013) 1981. [17] A.M. Lepadatu, I. Stavarache, T.F. Stoica, M.L. Ciurea, Dig. J. Nano Biomater. 6 (2011) 67–73. [18] K.H. Chiang, S.W. Lu, Y.H. Peng, C.H. Kuan, C.S. Tsai, J. Appl. Phys. 104 (2008) 014506. [19] C. Palade, A. Slav, A.M. Lepadatu, V.S. Teodorescu, M.L. Ciurea, Semiconductor Conference (CAS), 2014, International, IEEE, 2014 59–62. [20] J. Robertson, Eur. Phys. J. Appl. Phys. 28 (2004) 265–291. [21] N.C. Zoita, V. Braic, M. Danila, A.M. Vlaicu, C. Logofatu, C.E.A. Grigorescu, M. Braic, J. Cryst. Growth 389 (2014) 92–98. [22] C. Bonafos, M. Carrada, G. Benassayag, S. Schamm-Chardon, J. Groenen, V. Paillard, B. Pecassou, A. Claverie, P. Dimitrakis, E. Kapetanakis, V. Ioannou-Sougleridis, P. Normand, B. Sahu, A. Slaoui, Mater. Sci. Semicond. Process. 15 (2012) 615–626. [23] J.F. Moulder, W.F. Stickle, P.E. Sobol, K. Bomben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, second ed.Perkin-Elmer Corporation (Physical Electronics), 1992. [24] I. Stavarache, A.M. Lepadatu, N.G. Gheorghe, R.M. Costescu, G.E. Stan, D. Marcov, A. Slav, G. Iordache, T.F. Stoica, V. Iancu, V.S. Teodorescu, C.M. Teodorescu, M.L. Ciurea, J. Nanopart. Res. 13 (2011) 221–232. [25] H.-S. Jung, H.K. Kim, I.-H. Yu, S.Y. Lee, J. Lee, J. Park, J.H. Jang, S.-H. Jeon, Y.J. Chung, N.-I. Lee, T.J. Park, J.-H. Choi, C.S. Hwang, J. Electrochem. Soc. 159 (2012) G33–G39. [26] S. Kim, S.W. Hwang, S.-H. Choi, R.E. Elliman, Y.-M. Kim, Y.-J. Kim, J. Appl. Phys. 105 (2009) 106112. [27] S. Das, R.K. Singha, S. Gangopadhyay, A. Dhar, S.K. Ray, J. Appl. Phys. 108 (2010) 053510. [28] G. Chen, Z. Huo, L. Jin, D. Zhang, S. Zhao, Y. Han, S. Liu, M. Liu, Semicond. Sci. Technol. 29 (2014) 045019. [29] Y. Zhang, Y.Y. Shao, X.B. Lu, M. Zeng, Z. Zhang, X.S. Gao, X.J. Zhang, J.-M. Liu, J.Y. Dai, Appl. Phys. Lett. 105 (2014) 172902. [30] K.S. Kim, M.H. Jung, G.H. Park, W.J. Cho, J. Jung, J. Korean Phys. Soc. 5 (2009) 962–965. [31] L. Khomenkova, B.S. Sahu, A. Slaoui, F. Gourbilleau, Nanoscale Res. Lett. 6 (2011) 172. [32] S. Das, R.K. Singha, S. Manna, S. Gangopadhyay, A. Dhar, S.K. Ray, J. Nanopart. Res. 13 (2011) 587–595.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
How morphology determines the charge storage properties of Ge nanocrystals in HfO2 A. Slav a, C. Palade a,b, A.M. Lepadatu a,⁎, M.L. Ciurea a,c, V.S. Teodorescu a, S. Lazanu a, A.V. Maraloiu a, C. Logofatu a, M. Braic d, A. Kiss d a
National Institute of Materials Physics, 105 bis Atomistilor Street, 077125 Magurele, Romania University of Bucharest, Faculty of Physics, 405 Atomistilor Street, 077125 Magurele, Romania Academy of Romanian Scientists, 54 Splaiul Independentei, 050094 Bucuresti, Romania d National Institute for Optoelectronics, 409 Atomistilor Street, 077125 Magurele, Romania b c
a r t i c l e
i n f o
Article history: Received 1 October 2015 Accepted 13 October 2015 Available online xxxx Keywords: Nanostructured materials High-angle annular dark field (HAADF) High-resolution electron microscopy (HREM) X-ray photoelectron spectroscopy (XPS) Charge storage properties
a b s t r a c t The strong correlation between morphology and charge storage properties of HfO2/Ge/HfO2/Si trilayer structures was evidenced. The morphology of structures deposited by magnetron sputtering and electron beam evaporation was tailored by rapid thermal annealing and investigated by transmission electron microscopy, Raman and X-ray photoelectron spectroscopies. The best hysteresis loops (capacitance–voltage characteristics) were obtained for trilayers with high density Ge nanocrystals located in the position of as-deposited Ge layer. The decrease of Ge nanocrystals density narrows the memory window, by spreading Ge atoms into HfO2 matrix (sputtered trilayers), or by Ge atoms expulsion toward HfO2 nanocrystals surface (evaporated trilayers). © 2015 Elsevier Ltd. All rights reserved.
The memory properties of Si and Ge nanocrystals (NCs) embedded in various insulator matrices, including high-k oxides have been intensively studied for their applications in nonvolatile memories (NVMs) [1–10]. By using Ge NCs instead of Si NCs, the stronger quantum confinement effect [7] is exploited for improving the memory properties. Downscaling of device size conduced to the replacement of SiO2 as tunnel and/or gate oxides in MOS-like devices with a high-k dielectric such as HfO2 [11]. The most used deposition methods for obtaining MOS-like NVMs based on Ge NCs are magnetron sputtering (MS), electron beam evaporation (EBE), chemical vapour deposition and molecular beam epitaxy [8,11–13]. The size and density [14–15] of Ge NCs in the intermediate Ge layer positioned between tunnel and gate oxides together with the location [16] of intermediate layer are well controlled by using rapid thermal annealing (RTA). Also, RTA processing may induce traps at the Ge NCs/matrix interface and/or at oxide/Si interface [17–18]. Although the charge storage properties of structures with Ge NCs embedded in HfO2 matrix, prepared under different conditions were extensively investigated, their correlation with morphology was not systematically studied. So, in this paper the relationship between morphology and memory properties of structures based on Ge NCs embedded in HfO2 matrix is investigated. For this purpose, HfO2/Ge/HfO2/Si trilayer structures were deposited by using either MS or EBE methods, ⁎ Corresponding author. E-mail address: lepadatu@infim.ro (A.M. Lepadatu).
http://dx.doi.org/10.1016/j.scriptamat.2015.10.028 1359-6462/© 2015 Elsevier Ltd. All rights reserved.
and their morphology was tailored by subsequent RTA under different conditions. The morphology of trilayers was studied by using highresolution transmission electron microscopy (HRTEM), high-angle annular dark field-scanning TEM (HAADF-STEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The charge storage properties were investigated by measuring capacitance–voltage (C–V) at different frequencies and capacitance–time (C–t) characteristics. We show that the only contributors to the charge storage properties of our trilayer structures are Ge NCs with high density located in the position of as-deposited Ge layer. We prepared trilayer structures (MOS-like capacitors) with Ge NCs embedded in HfO2 on p-type Si (100) wafers with 7–14 Ωcm resistivity. Si wafers were classically cleaned in wet chemical solution, followed by dipping in diluted HF solution (2%) for removing the native Si oxide. The trilayers were deposited by MS in pure Ar (6N) of 4 mTorr pressure, by using 40–60 W RF for HfO2 and 5–10 W DC for Ge. So, the tunnel HfO2 layer has 5–10 nm thickness, the intermediate Ge layer has 5–10 nm and the gate HfO2 layer 20–50 nm. For EBE deposition, layers with similar thicknesses were prepared by using a rate of about 0.025 nm/s for HfO2 and 0.054 nm/s for Ge. We also prepared control capacitors (MOS capacitors) without Ge content using similar conditions to the trilayer capacitors. All MS and EBE structures (including control ones) were deposited on Si wafers kept at room temperature. All as-deposited trilayers are amorphous, therefore for Ge nanostructuring they were annealed in a rapid thermal processor in N2 atmosphere (6N), at different temperatures between 600 and 1000
136
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
°C for 3–9 min. The as-deposited control structures are also amorphous and after RTA performed similarly to the trilayer structures, HfO2 becomes crystallized [19]. The MOS-like capacitors were obtained by thermal evaporation of Al contacts with area of 6.4 × 10−3 cm2 on the top and backside of the trilayer structures. The morphology of trilayer structures was investigated by using a Jeol ARM 200F electron microscope, a confocal T64000 Horiba Jobin Yvon Raman spectrometer (514 nm Ar laser) and a SPECS XPS spectrometer (Phoibos 150 MCD electron energy analyser, constant energy mode, monochromatic X-ray radiation Al Kα 1486.74 eV). Fig. 1(a, b and c) presents HRTEM images of MS trilayers showing the cross-section morphology evolution from the as-deposited structures to structures annealed at 600 and 850 °C for 8 min (MS-600 and MS-850, respectively). As one can see, the as-deposited trilayer structure is amorphous. MS-600 sample is crystallized and keeps the trilayer morphology, the intermediate Ge layer being visible. For MS-850 structure, the trilayer morphology is less visible and HfO2 is crystallized all over the structure thickness. At the interface of deposited layers with the Si substrate an amorphous SiO2 layer grown during deposition [20–21] is visible, that is thicker in MS-850 sample than in MS-600. The HAADF-STEM and HRTEM images of the cross-section of MS600 structures are compared in Fig. 1(d and e). The Z contrast in the HAADF image reveals the presence of a row of Ge nanoparticles (NPs) with sizes of 5–7 nm, located in the initial position of as-deposited Ge layer. The Z contrast shows only the agglomeration of Ge atoms (in NPs) that can be or not in an oxidation state. The Ge NCs embedded in crystalline HfO2 cannot be evidenced in TEM [22], due to their similar lattice constants. In Fig. 1(f), the Raman spectrum measured on MS-600 together with the fit curve using Richter phonon quantum confinement model [16] are presented. The peak located at 299.2 cm−1 corresponds to Ge NCs with 6.5 nm average size, showing that the Ge NPs evidenced in microscopy are crystallized. Fig. 1(g) shows XPS spectra of Ge 2p3/2 measured on MS-600 trilayers at different depths under the free surface after trilayer sputtering (Ar+) with about 0.5 nm/min rate. In the curve taken at the free surface, the peak located at 1220.6 eV corresponds to fully oxidized Ge (GeO2) [23]. The first two sputtering steps (1 and 2 at about 2.5 and 5 nm depths, respectively) do not evidence any Ge signal. Then, the curves obtained after steps 3, 4 and 5 (22.5, 25 and 27.5 nm depths) are formed of two bands, one band corresponds to metallic Ge positioned at 1217 eV [23] and the other one can be attributed to partially oxidized Ge [24–25]. In Refs. [26–27], metallic Ge and partially oxidized Ge were evidenced in Ge 3d band. In Fig. 1(g), for steps 3, 4 and 5, the ratios of the intensities of metallic and oxidized Ge peaks are 42/58, 60/40 and 44/56, respectively. In the curve obtained after step 4 (at depth corresponding to intermediate layer of Ge NCs) the metallic Ge contribution overcomes the contribution of oxidized Ge (60/40). The decrease of intensities ratio after step 5 (44/56) shows the decrease of metallic Ge amount with respect to oxidized Ge as the sputtering depth is close to the tunnelling HfO2 layer depth. In Fig. 2, HRTEM and HAADF-STEM images taken on EBE-850 and EBE-900 structures annealed for 4 min are shown. In EBE-850 structures (Fig. 2(a and b)), Ge NPs of 5–6 nm size are located in the position of asdeposited Ge layer, similarly to MS-600 structure. Ge NPs are also crystallized as Raman measurements reveal. In EBE-900 (Fig. 2(c and d)), the trilayer morphology is lost and the sample contains HfO2 NCs that are extended over all the trilayer thickness (HRTEM image). Ge NPs with sizes of 5–6 nm are visible in the bottom part of the HfO2 NCs layer and in between the HfO2 NCs (HAADF image). The growth process of HfO2 NCs corresponding to 900 °C annealing eliminates Ge atoms from the HfO2 lattice toward HfO2 NCs surface. Therefore, the Ge atoms segregate either at the bottom part of HfO2 NCs i.e. on the top of SiO2 layer or in between the HfO2 NCs, while Ge atoms arriving at the free surface are either oxidized or lost.
Fig. 1. HRTEM images of MS HfO2/Ge/HfO2/Si trilayers — cross-section morphology evolution from (a) as-deposited to (b) MS-600 and (c) MS-850 structures; (d)–(g) measurements on MS-600: (d) HAADF-STEM and (e) HRTEM cross-sectional images; (f) experimental and fit Raman spectra and (g) XPS spectra and deconvolution curves at the free surface and after different sputtering steps (1, 2, 3, 4 and 5).
The memory characteristics of MOS-like capacitors based on the trilayer structures with the morphologies described above are presented in the following. C–V characteristics were measured in the dark at room temperature (Agilent E4980A LCR Meter).
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
137
Fig. 2. HRTEM and HAADF-STEM images of the same area of EBE-850 (a and b) and EBE-900 structures (c and d).
In Fig. 3(a), the C–V characteristics recorded on the capacitors based on MS-600 trilayers (MS-600 capacitors) in the voltage range of (−4,+4) V, and at frequencies of 1 MHz, 500 and 100 kHz are given. The C–V characteristics show a hysteresis with a memory window ΔV of about 1 V that is frequency-independent meaning that the hysteresis loop is due to Ge NCs [18]. The inset contains the charge retention curve (C–t), taken on the MS-600 capacitor. For this, firstly the capacitor was charged for 3 min at − 4 V, then the voltage was changed to the flatband voltage, and then the C–t curve was recorded. After 4000 s, the capacitance decreased with 28%. In Fig. 3(b), C–V characteristics measured at 1 MHz, on MS-600 and MS-850 capacitors and also on the HfO2-600 control capacitor (without Ge) are shown. One can see that the control capacitor exhibits no memory window although in literature, important memory effects were reported on HfO2 charge trapping flash memories [28–30]. Therefore, in our structures, the traps present in HfO2 do not contribute to charge storage. The increase of annealing temperature from 600 to 850 °C produces the decrease of ΔV to 0.2 V (MS-850 capacitors), and the appearance of a small hump at positive voltages (inset) that is the signature of slow traps located at the Si/SiO2 interface [31]. Similarly to MS-600, ΔV
is independent of frequency. The MS capacitors annealed below 600 °C present narrower hysteresis loops. C–V measurements were also performed on EBE capacitors in the (−4,+4) V voltage interval at frequencies of 1 MHz, 600 and 100 kHz. The C–V curves obtained on the EBE-850 capacitor are shown in Fig. 3(c). They have a memory window ΔV of 0.8 V that is also frequency-independent as for MS-600 capacitors, the hysteresis being also due to Ge NCs. The C–V characteristics taken on control capacitors are similar to MS case, namely they exhibit no memory window. Additionally, the hysteresis loops shift toward more negative voltages when the frequency increases, which points to the presence of fast traps possibly located at the interface between the Ge NCs and HfO2 matrix [18]. This frequency-dependent shift has not been observed in MS capacitors (Fig. 3(a)). All hysteresis curves are centred on negative voltages (Fig. 3(c)). By increasing the annealing temperature from 850 to 1000 °C, they additionally shift toward more negative voltages and ΔV decreases, as can be seen in Fig. 3(d). Also, ΔV is independent of frequency for EBE-900 and EBE-1000 capacitors. For RTA below 850 °C, narrower memory windows were obtained. The C–t curve taken on EBE-850 is also represented in the inset, showing a decrease of 25% after 4000 s.
Fig. 3. (a) C–V characteristics of MS-600 capacitors measured at different frequencies; inset — C–t characteristic; (b) C–V characteristics of MS-600, MS-850 and control (HfO2-600) capacitors; inset — detail of MS-850 curve. (c) C–V characteristics of EBE-850 capacitors measured at different frequencies; inset — C–t and (d) C–V characteristics of EBE-850, EBE-900 and EBE-1000 capacitors.
138
A. Slav et al. / Scripta Materialia 113 (2016) 135–138
The best hysteresis loops were obtained on the MS-600 (8 min RTA) and EBE-850 (4 min RTA) capacitors. In our opinion, they are due only to the Ge NCs embedded in HfO2 as it results from the electrical behaviour of capacitors, i.e. frequency-independence of memory windows [18,32] corroborated with no memory effect in capacitors without Ge content. This is also supported by XPS (Ge located in the position of asdeposited layer is in metallic form) and by Raman spectrum (Ge NCs of 6.5 nm size) for MS samples. Therefore, the Ge NPs observed in HRTEM and HAADF-STEM images are crystallized (Fig. 1(b, d and e) for MS-600 and Fig. 2(a and b) for EBE-850). In the case of MS structures, the increase of the annealing temperature produces Ge spreading into the crystallized HfO2 matrix (Fig. 1(c)). The formation of Ge NCs is hindered by Ge fast diffusion in the HfO2 lattice [16] that leads to the decrease of Ge NCs density and therefore to the narrowing of the memory window (Fig. 3(b)). In EBE structures, the increase of the annealing temperature leads to the segregation of Ge atoms at the surface of HfO2 NCs (at the HfO2/SiO2 interface or at the interface between HfO2 NCs — Fig. 2(c and d)). In other words, the crystal growth process of HfO2 NCs during annealing at 900 °C eliminates the Ge atoms from the HfO2 lattice toward HfO2 NCs surface, and thus the density of Ge NCs located inside HfO2 NCs diminishes, and therefore ΔV decreases. It results that the best memory characteristics found in MS-600 and EBE-850 capacitors are due to the high density of Ge NCs embedded in HfO2 matrix, located in the initial position of Ge layer. By tailoring the morphology of trilayers through the increase of annealing temperature, the hysteresis loops are narrowed as a result of Ge atoms spreading into the HfO2 matrix (in MS-850) or of their segregation at HfO2 NCs surface (in EBE-900). In summary, we have shown the determinant influence of morphology on charge storage properties of HfO2/Ge/HfO2/Si trilayer structures. So, by tailoring the morphology, the memory properties of these trilayers are tuned. The investigated structures were deposited by MS and EBE methods and nanostructured by RTA under different conditions. We have found that the best hysteresis loops are obtained on capacitors with similar morphologies, namely on MS-600 (8 min RTA) and EBE-850 (4 min RTA). In these trilayer structures, Ge NCs embedded in crystallized HfO2 are located in the position of as-deposited Ge layer, have a high density and are the only contributors to the charge storage properties. By changing the morphology of trilayers with the increase of annealing temperature, the density of Ge NCs is decreased producing a narrowing of the memory windows. So, in MS trilayer structures the Ge NCs formation is hindered due to the spreading of Ge atoms into the HfO2 matrix, while in EBE structures Ge atoms are expulsed from the HfO2 lattice during crystal growth process toward HfO2 NCs surface. The understanding of the relationship between morphology and charge storage properties is useful for the preparation of materials with improved properties for NVMs applications. Acknowledgements The authors would like to thank Prof. Ioan Baltog for the helpful discussions related to Raman results. This work was supported by the Romanian National Authority for Scientific Research through the
CNCS-UEFISCDI Contracts No. PN II-PT-PCCA-9/2012 and No. PNII-ID/ 289-2011. References [1] T.-C. Chang, F.-Y. Jian, S.-C. Chen, Y.-T. Tsai, Mater. Today 14 (2011) 608–615. [2] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbe, K. Chan, Appl. Phys. Lett. 68 (1996) 1377–1379. [3] G. Gay, G. Molas, M. Bocquet, E. Jalaguier, M. Gély, L. Masarotto, J.P. Colonna, H. Grampeix, F. Martin, P. Brianceau, V. Vidal, R. Kies, T. Baron, G. Ghibaudo, B. De Salvo, IEEE Trans. Electron Devices 59 (2012) 933–940. [4] J.S. de Sousa, R. Peibst, M. Erenburg, E. Bugiel, G.A. Farias, J.P. Leburton, K.R. Hofmann, IEEE Trans. Electron Devices 58 (2011) 376–383. [5] J. Martín-Sánchez, A. Chahboun, M.J.M. Gomes, A.G. Rolo, B. Pivac, I. Capan, Phys. Status Solidi (RRL) 6 (2012) 223–225. [6] R. Gupta, W.J. Yoo, Y. Wang, Z. Tan, G. Samudra, S. Lee, D.S.H. Chan, K.P. Loh, L.K. Bera, N. Balasubramanian, D.-L. Kwong, Appl. Phys. Lett. 84 (2004) 4331–4433. [7] S.K. Ray, S. Maikap, W. Banerjee, S. Das, J. Phys. D: Appl. Phys. 46 (2013) 153001. [8] S. Wang, W. Lu, Q. Wan, J.Y. Dai, P.F. Lee, L. Suhua, Q. Shen, M. Zhang, Z. Song, C. Lin, Appl. Phys. Lett. 86 (2005) 113105. [9] B.S. Sahu, F. Gloux, A. Slaoui, M. Carrada, D. Muller, J. Groenen, C. Bonafos, S. Lhostis, Nanoscale Res. Lett. 6 (2011) 177. [10] D. Lehninger, P. Seidel, M. Geyer, F. Schneider, V. Klemm, D. Rafaja, J. von Borany, J. Heitmann, Appl. Phys. Lett. 106 (2015) 023116. [11] S. Das, K. Das, R.K. Singha, A. Dhar, S.K. Ray, Appl. Phys. Lett. 91 (2007) 233118. [12] Y.Q. Wang, J.H. Chen, W.J. Yoo, Y.-C. Yeo, Mater. Res. Soc. Symp. Proc. 830 (2005) 269–274. [13] R. Aluguri, S. Das, R.K. Singha, S.K. Ray, Curr. Appl. Phys. 13 (2013) 12–17. [14] L.W. Teo, W.K. Choi, W.K. Chim, V. Ho, C.M. Moey, M.S. Tay, C.L. Heng, Y. Lei, D.A. Antoniadis, E.A. Fitzgerald, Appl. Phys. Lett. 81 (2002) 3639–3941. [15] C.G. Ahn, H.S. Kang, Y.K. Kwon, S.M. Lee, B.R. Ryum, B.K. Kang, J. Appl. Phys. 86 (1999) 1542–1547. [16] A.-M. Lepadatu, T. Stoica, I. Stavarache, V.S. Teodorescu, D. Buca, M.L. Ciurea, J. Nanopart. Res. 15 (2013) 1981. [17] A.M. Lepadatu, I. Stavarache, T.F. Stoica, M.L. Ciurea, Dig. J. Nano Biomater. 6 (2011) 67–73. [18] K.H. Chiang, S.W. Lu, Y.H. Peng, C.H. Kuan, C.S. Tsai, J. Appl. Phys. 104 (2008) 014506. [19] C. Palade, A. Slav, A.M. Lepadatu, V.S. Teodorescu, M.L. Ciurea, Semiconductor Conference (CAS), 2014, International, IEEE, 2014 59–62. [20] J. Robertson, Eur. Phys. J. Appl. Phys. 28 (2004) 265–291. [21] N.C. Zoita, V. Braic, M. Danila, A.M. Vlaicu, C. Logofatu, C.E.A. Grigorescu, M. Braic, J. Cryst. Growth 389 (2014) 92–98. [22] C. Bonafos, M. Carrada, G. Benassayag, S. Schamm-Chardon, J. Groenen, V. Paillard, B. Pecassou, A. Claverie, P. Dimitrakis, E. Kapetanakis, V. Ioannou-Sougleridis, P. Normand, B. Sahu, A. Slaoui, Mater. Sci. Semicond. Process. 15 (2012) 615–626. [23] J.F. Moulder, W.F. Stickle, P.E. Sobol, K. Bomben, in: J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, second ed.Perkin-Elmer Corporation (Physical Electronics), 1992. [24] I. Stavarache, A.M. Lepadatu, N.G. Gheorghe, R.M. Costescu, G.E. Stan, D. Marcov, A. Slav, G. Iordache, T.F. Stoica, V. Iancu, V.S. Teodorescu, C.M. Teodorescu, M.L. Ciurea, J. Nanopart. Res. 13 (2011) 221–232. [25] H.-S. Jung, H.K. Kim, I.-H. Yu, S.Y. Lee, J. Lee, J. Park, J.H. Jang, S.-H. Jeon, Y.J. Chung, N.-I. Lee, T.J. Park, J.-H. Choi, C.S. Hwang, J. Electrochem. Soc. 159 (2012) G33–G39. [26] S. Kim, S.W. Hwang, S.-H. Choi, R.E. Elliman, Y.-M. Kim, Y.-J. Kim, J. Appl. Phys. 105 (2009) 106112. [27] S. Das, R.K. Singha, S. Gangopadhyay, A. Dhar, S.K. Ray, J. Appl. Phys. 108 (2010) 053510. [28] G. Chen, Z. Huo, L. Jin, D. Zhang, S. Zhao, Y. Han, S. Liu, M. Liu, Semicond. Sci. Technol. 29 (2014) 045019. [29] Y. Zhang, Y.Y. Shao, X.B. Lu, M. Zeng, Z. Zhang, X.S. Gao, X.J. Zhang, J.-M. Liu, J.Y. Dai, Appl. Phys. Lett. 105 (2014) 172902. [30] K.S. Kim, M.H. Jung, G.H. Park, W.J. Cho, J. Jung, J. Korean Phys. Soc. 5 (2009) 962–965. [31] L. Khomenkova, B.S. Sahu, A. Slaoui, F. Gourbilleau, Nanoscale Res. Lett. 6 (2011) 172. [32] S. Das, R.K. Singha, S. Manna, S. Gangopadhyay, A. Dhar, S.K. Ray, J. Nanopart. Res. 13 (2011) 587–595.