- Email: [email protected]
embedded in Al2O3 gate dielectric
Applied Surface Science 230 (2004) 8–11
Short communication
Synthesis and electron storage characteristics of isolated silver nanodots on/embedded in Al2O3 gate dielectric Q. Wanga,b,*, Z.T. Songa, W.L. Liua, C.L. Lina, T.H. Wangb a
The Research Center of Semiconductor Functional Film Engineering Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China b Institute of Physics, Chinese Academy of Sciences, Beijing, China Received in revised form 7 August 2003; accepted 15 February 2004 Available online 26 April 2004
Abstract Monolayer-isolated silver (Ag) nanodots with the average diameter down to 7 nm are synthesized on Al2O3/Si substrate by vacuum electron-beam evaporation followed by annealing at 400 8C in N2 ambient. Metal–insulator-silicon (MIS) structures with Ag nanodots embedded in Al2O3 gate dielectric are fabricated. Clear electron storage effect with the flatband voltage shift of 1.3 eV is observed through capacitance–conductance and conductance–voltage measurements. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystal floating-gate memory devices. # 2004 Elsevier B.V. All rights reserved. PACS: 61.46.þw; 81.16.Dn; 81.07.Ta; 73.63.Bd Keywords: Ag nanodots; Al2O3 gate dielectrics; Electron storage effect
A nanocrystal floating gate memory is traditionally a metal-oxide-semiconductor field-effect transistor (MOSFET) in which the gate dielectric is replaced by a gate stack consisting of a thin tunneling oxide, a layer of semiconductor nanocrystals embedded in dielectric, and a thicker oxide. Such memory devices have been recently proposed and widely investigated in industrial and academic laboratories [1–3]. But up to now, for nanocrystal floating gate memory fabrication, most investigations just focus on Si or Ge nanocrystals embedded in SiO2 [4–7]. In fact,
*
Corresponding author. Tel.: þ86-21-62511070; fax: þ86-21-62513510. E-mail address: [email protected] (Q. Wang).
aluminum oxide (Al2O3) is a promising candidate as gate dielectric [8,9]. For most high-k dielectrics, higher dielectric constant usually comes at the expense of narrower band gap. With a dielectric constant (eAl2 O3 9) more than twice that of SiO2 (eSiO2 3:9), Al2O3 has the band gap of about 9 eV. Al2O3 film is also a robust, high-temperature material expected to withstand Si processing conditions. At the same time, metal nanocrystals have many advantages over the semiconductor counterparts including higher density of states around the Fermi level, stronger coupling with the conduction channel, a wide range of available work functions, and smaller energy perturbation due to carrier confinement [10]. Metal nanocrystals also provide a great degree of scalability for the nanocrystals size.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.045
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11
Based on these facts, in this letter, vacuum electronbeam evaporation method [11] followed by annealing in N2 ambient is used to synthesize high-density isolated silver (Ag) nanodots on Al2O3/Si substrate. Metal–insulator-silicon (MIS) structures containing Ag nanocrystals embedded in Al2O3 dielectrics are fabricated. Electron storage effect of the Ag nanocrystals is experimentally manifested by the hysteresis in the C–V curves. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystal floating-gate memory. Solid Al2O3 and Ag with high purity (>99.99%) were used as the evaporation sources in our experiment. The temperature of the Si substrate was about 300 K. First, Al2O3 film with the thickness of 2.5 nm was deposited on hydrofluoric treated n-type silicon substrate with doping density of 1015 cm3. Then, Ag films with the thickness of 10 and 3 nm were deposited on the Al2O3 layer, respectively. After film deposition, annealing was performed at 400 8C in N2 ambient for 1 h. Detailed morphological characterization and statistical analysis were performed ex situ by atomic force microscopy (AFM, Digital Instruments Nanoscope III) in tapping mode and scanning electron microscopy (SEM, JEOL-JSM-6700F). Fig. 1(a) shows the AFM image of the as deposited 10 nm Ag on Al2O3/Si. The surface morphology is very smooth and nanoclusters with ultra-high density are observed. The size of the Ag nanocrystrals is less than 10 nm. It is well known that there are three known modes of heteroepitaxial growth: Frank–van der Merwe (FvdM), Volmer–Weber (VW), and Stranski– Krastanov (SK). [12] Generally, in the case of deposition of ultra thin metal film on amorphous oxide, the interface energy is sufficient to cause island formation, VW growth modes will occur. No wetting lay is needed and direct nucleation of three-dimensional nanocrystals takes place. Fig. 1(b) shows the SEM image of the 400 8C annealed sample. Isolated Ag nanodots in the form of monolayer with the mean diameter size of about 30 nm are observed in the image. Annealing gives the Ag atoms enough surface mobility, and Ag nanoclusters film will self-assemble into a lower-total-energy state. The Ostwals ripening mechanism, [13] which describes the growth of larger dots at the expense of smaller dots, can explain the selfassembling phenomenon of Ag nanodots on Al2O3/Si. A clear different in density and size between as depos-
9
Fig. 1. (a) AFM image of the 10 nm Ag film on Al2O3/Si substrate, deposited at room temperature, and (b) SEM image of the sample annealed at 400 8C in N2 ambient for 1 h.
ited sample and the annealed sample is due to the rapid coarsening process of the Ag during the annealing process. Once the Ag nanodots have formed, the work function difference between the metal and the Si substrate generates localized depletion or accumulation region in the substrate. [10] The repulsion force between those regions is of great help to form isolated Ag nanodots with a definite distance between them. Fig. 2(a) shows the SEM image of the annealed sample of 3 nm Ag on Al2O3/Si substrate. The mean
10
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11 1.2
(a)
500K Hz
1.0
C/Cox
0.8 0.6 0.4 0.2 0.0 -3
-2
-1
0
1
2
3
Voltage (V)
0.25
Conductance (a.u)
(b) 0.20 0.15 0.10 0.05 0.00 -3
-2
-1
0
1
2
3
Voltage (V)
Fig. 3. (a) C–V and (b) G–V characteristics obtained by sweeping gate voltage between 3 and þ3 V. The peak position in the G–V curve is around the flatband voltage.
Fig. 2. (a) SEM image of the Ag film with the thickness of 3 nm on Al2O3/Si substrate, annealed at 400 8C in N2 ambient for 1 h. (b) Schematic illustration of the metal–insulator-silicon (MIS) structures containing Ag nanodots embedded in Al2O3 dielectrics.
diameter of the Ag nanodots is about 7 nm and the density is about 7 1011 cm2. These high-density, isolated Ag nanodots are of great significance for nanocrystal floating-gate memory application. Control gate oxide of Al2O3 with the thickness of 5 nm was deposited on Ag nanodots by vacuum electron-beam evaporation for MIS structures fabrication. Metal electrodes were formed by evaporation of aluminum (Al) though the shadow masks and the their area is about 2 102 cm2. For comparison, a reference sample of Al/Al2O3/Si MIS structure without Ag nanodtos was also fabricated. Post-annealing was performed at 400 8C in N2 ambient for 1 h. Fig. 2(b)
shows schematic illustration of the fabricated MIS structure. The electrical characteristics were investigated with HP 4284A precision LCR meter at 500 kHz to study the electron storage effect of the Ag nanodots. The C–V and G–V characteristics shown in Fig. 3 were measured by sweeping the voltage between inversion and accumulation regions at room temperature. A clockwise hysteresis is observed in both C–V and G–V curves. We also observed a conductance peak in G–V curve, which is close to the flatband voltage, in either the forward or backward sweep direction. Moreover, no distortions in the C–V curves due to deep defect traps or large interface state density were observed. Both the magnitude of the hysteresis in C–V and the shift of the peak in G–V are about 1.3 V. No notable C–V hysteresis is observed for the reference MIS structure without Ag nanodots, implying that this hysteresis is Ag nanodots related. Generally, the hysteresis, also called memory effects
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11
can be explained by the injected charges stored in nanocrystals or at the interface of the nanocrystals. The hysteresis is clockwise, indicating net negative charging, which should be due to electron trapping in the Ag nanodots. The large work function of Ag (about 4.3 eV) will create an asymmetrical barrier between the Si substrate and the storage nanodots, i.e., a small barrier for writing and a large barrier for retention [10], which gives much freedom for device optimization. After electrons are stored, at a chosen reading voltage (0.75 V, the flat-band voltage in our experiment) the trapped electrons have a probability to tunnel back to the Si substrate, which would cause a gradual shift in capacitance [14]. Time dependent capacitance measurements were performed to study the charge-retention characteristics of the Ag nanodots. If we defined the time for the loss of 10% of the stored electron as the retention time, the retention time of our device was found to exceed 8 h. The asymmetrical barrier between the Ag nanodots and the substrate (a lower barrier for writing and a higher barrier for retention) is of great advantage for the long retention time, as compared to semiconductor nanocrystals. In summary, we have demonstrated a synthesis method, which is capable of synthesis a monolayer of metal (Ag in this letter) nanodots on/embedded in Al2O3/Si substrate. Ag nanodots with the average diameter of 7 nm and the density of 7 1011 cm2 can be obtained. MIS structures using Al2O3 as gate dielectric embedded with Ag nanodots are fabricated. Clear electron storage effects are observed through the C–V and G–V measurements. A long charge-retention time is obtained due to the asymmetrical barrier between the Ag nanodots and the substrate. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystals floating-gate memory device.
11
Acknowledgements The authors acknowledge the support from the Special Funds for Major State Basic Research Project No. G2001CB3095 of China and the National Natural Science Foundation of China through Grant Nos. 69925410, 60201004 and also the support from Shanghai Nanotechnology Promotion Center (No.0252nm084 and No.0359nm004).
References [1] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbe, K. Chan, Appl. Phys. Lett. 68 (1996) 1377. [2] S. Tiwari, F. Rana, K. Chan, L. Shi, H. Hanafi, Appl. Phys. Lett. 69 (1996) 1232. [3] M.L. Ostraat, J.W. De Blauwe, M.L. Green, L.D. Bell, M.L. Brongersma, J. Casperson, R.C. Flagan, H.A. Atwater, Appl. Phys. Lett. 79 (2001) 433. [4] W.K. Choi, W.K. Chim, C.L. Heng, L.W. Teo, D.A. Antoniadis, E.A. Fitzgerald, Appl. Phys. Lett. 80 (2002) 2014. [5] 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. [6] D.N. Kouvatsos, V. Loannou-sougleridis, G. Nassiopoulou, Appl. Phys. Lett. 82 (2003) 397. [7] G.B. Assayag, C. Bonafos, M. Carrada, A. Claverie, Appl. Phys. Lett. 82 (2003) 200. [8] E.P. Gusev, M. Copel, E. Cartier, I.J.R. Baumvol, C. Krug, M.A. Gribelyuk, Appl. Phys. Lett. 76 (2000) 176. [9] M. Kundu, N. Miyata, M. Ichikawa, J. Appl. Phys. 93 (2003) 1498. [10] Z. Liu, C. Lee, V. Narayanan, G. Pei, E.C. Kan, IEEE Trans. Electr. Devices 49 (2002) 1606. [11] Q. Wan, N.L. Zhang, X.Y. Xie, T.H. Wang, C.L. Lin, Appl. Surf. Sci. 191 (2002) 171. [12] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [13] W. Ostwald, Z. Phys. Chem. (Leipzig) 34 (1900) 495. [14] S. Huang, S. Banerjee, R.T. Tung, S. Oda, J. Appl. Phys. 93 (2003) 576.
Short communication
Synthesis and electron storage characteristics of isolated silver nanodots on/embedded in Al2O3 gate dielectric Q. Wanga,b,*, Z.T. Songa, W.L. Liua, C.L. Lina, T.H. Wangb a
The Research Center of Semiconductor Functional Film Engineering Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China b Institute of Physics, Chinese Academy of Sciences, Beijing, China Received in revised form 7 August 2003; accepted 15 February 2004 Available online 26 April 2004
Abstract Monolayer-isolated silver (Ag) nanodots with the average diameter down to 7 nm are synthesized on Al2O3/Si substrate by vacuum electron-beam evaporation followed by annealing at 400 8C in N2 ambient. Metal–insulator-silicon (MIS) structures with Ag nanodots embedded in Al2O3 gate dielectric are fabricated. Clear electron storage effect with the flatband voltage shift of 1.3 eV is observed through capacitance–conductance and conductance–voltage measurements. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystal floating-gate memory devices. # 2004 Elsevier B.V. All rights reserved. PACS: 61.46.þw; 81.16.Dn; 81.07.Ta; 73.63.Bd Keywords: Ag nanodots; Al2O3 gate dielectrics; Electron storage effect
A nanocrystal floating gate memory is traditionally a metal-oxide-semiconductor field-effect transistor (MOSFET) in which the gate dielectric is replaced by a gate stack consisting of a thin tunneling oxide, a layer of semiconductor nanocrystals embedded in dielectric, and a thicker oxide. Such memory devices have been recently proposed and widely investigated in industrial and academic laboratories [1–3]. But up to now, for nanocrystal floating gate memory fabrication, most investigations just focus on Si or Ge nanocrystals embedded in SiO2 [4–7]. In fact,
*
Corresponding author. Tel.: þ86-21-62511070; fax: þ86-21-62513510. E-mail address: [email protected] (Q. Wang).
aluminum oxide (Al2O3) is a promising candidate as gate dielectric [8,9]. For most high-k dielectrics, higher dielectric constant usually comes at the expense of narrower band gap. With a dielectric constant (eAl2 O3 9) more than twice that of SiO2 (eSiO2 3:9), Al2O3 has the band gap of about 9 eV. Al2O3 film is also a robust, high-temperature material expected to withstand Si processing conditions. At the same time, metal nanocrystals have many advantages over the semiconductor counterparts including higher density of states around the Fermi level, stronger coupling with the conduction channel, a wide range of available work functions, and smaller energy perturbation due to carrier confinement [10]. Metal nanocrystals also provide a great degree of scalability for the nanocrystals size.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.045
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11
Based on these facts, in this letter, vacuum electronbeam evaporation method [11] followed by annealing in N2 ambient is used to synthesize high-density isolated silver (Ag) nanodots on Al2O3/Si substrate. Metal–insulator-silicon (MIS) structures containing Ag nanocrystals embedded in Al2O3 dielectrics are fabricated. Electron storage effect of the Ag nanocrystals is experimentally manifested by the hysteresis in the C–V curves. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystal floating-gate memory. Solid Al2O3 and Ag with high purity (>99.99%) were used as the evaporation sources in our experiment. The temperature of the Si substrate was about 300 K. First, Al2O3 film with the thickness of 2.5 nm was deposited on hydrofluoric treated n-type silicon substrate with doping density of 1015 cm3. Then, Ag films with the thickness of 10 and 3 nm were deposited on the Al2O3 layer, respectively. After film deposition, annealing was performed at 400 8C in N2 ambient for 1 h. Detailed morphological characterization and statistical analysis were performed ex situ by atomic force microscopy (AFM, Digital Instruments Nanoscope III) in tapping mode and scanning electron microscopy (SEM, JEOL-JSM-6700F). Fig. 1(a) shows the AFM image of the as deposited 10 nm Ag on Al2O3/Si. The surface morphology is very smooth and nanoclusters with ultra-high density are observed. The size of the Ag nanocrystrals is less than 10 nm. It is well known that there are three known modes of heteroepitaxial growth: Frank–van der Merwe (FvdM), Volmer–Weber (VW), and Stranski– Krastanov (SK). [12] Generally, in the case of deposition of ultra thin metal film on amorphous oxide, the interface energy is sufficient to cause island formation, VW growth modes will occur. No wetting lay is needed and direct nucleation of three-dimensional nanocrystals takes place. Fig. 1(b) shows the SEM image of the 400 8C annealed sample. Isolated Ag nanodots in the form of monolayer with the mean diameter size of about 30 nm are observed in the image. Annealing gives the Ag atoms enough surface mobility, and Ag nanoclusters film will self-assemble into a lower-total-energy state. The Ostwals ripening mechanism, [13] which describes the growth of larger dots at the expense of smaller dots, can explain the selfassembling phenomenon of Ag nanodots on Al2O3/Si. A clear different in density and size between as depos-
9
Fig. 1. (a) AFM image of the 10 nm Ag film on Al2O3/Si substrate, deposited at room temperature, and (b) SEM image of the sample annealed at 400 8C in N2 ambient for 1 h.
ited sample and the annealed sample is due to the rapid coarsening process of the Ag during the annealing process. Once the Ag nanodots have formed, the work function difference between the metal and the Si substrate generates localized depletion or accumulation region in the substrate. [10] The repulsion force between those regions is of great help to form isolated Ag nanodots with a definite distance between them. Fig. 2(a) shows the SEM image of the annealed sample of 3 nm Ag on Al2O3/Si substrate. The mean
10
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11 1.2
(a)
500K Hz
1.0
C/Cox
0.8 0.6 0.4 0.2 0.0 -3
-2
-1
0
1
2
3
Voltage (V)
0.25
Conductance (a.u)
(b) 0.20 0.15 0.10 0.05 0.00 -3
-2
-1
0
1
2
3
Voltage (V)
Fig. 3. (a) C–V and (b) G–V characteristics obtained by sweeping gate voltage between 3 and þ3 V. The peak position in the G–V curve is around the flatband voltage.
Fig. 2. (a) SEM image of the Ag film with the thickness of 3 nm on Al2O3/Si substrate, annealed at 400 8C in N2 ambient for 1 h. (b) Schematic illustration of the metal–insulator-silicon (MIS) structures containing Ag nanodots embedded in Al2O3 dielectrics.
diameter of the Ag nanodots is about 7 nm and the density is about 7 1011 cm2. These high-density, isolated Ag nanodots are of great significance for nanocrystal floating-gate memory application. Control gate oxide of Al2O3 with the thickness of 5 nm was deposited on Ag nanodots by vacuum electron-beam evaporation for MIS structures fabrication. Metal electrodes were formed by evaporation of aluminum (Al) though the shadow masks and the their area is about 2 102 cm2. For comparison, a reference sample of Al/Al2O3/Si MIS structure without Ag nanodtos was also fabricated. Post-annealing was performed at 400 8C in N2 ambient for 1 h. Fig. 2(b)
shows schematic illustration of the fabricated MIS structure. The electrical characteristics were investigated with HP 4284A precision LCR meter at 500 kHz to study the electron storage effect of the Ag nanodots. The C–V and G–V characteristics shown in Fig. 3 were measured by sweeping the voltage between inversion and accumulation regions at room temperature. A clockwise hysteresis is observed in both C–V and G–V curves. We also observed a conductance peak in G–V curve, which is close to the flatband voltage, in either the forward or backward sweep direction. Moreover, no distortions in the C–V curves due to deep defect traps or large interface state density were observed. Both the magnitude of the hysteresis in C–V and the shift of the peak in G–V are about 1.3 V. No notable C–V hysteresis is observed for the reference MIS structure without Ag nanodots, implying that this hysteresis is Ag nanodots related. Generally, the hysteresis, also called memory effects
Q. Wang et al. / Applied Surface Science 230 (2004) 8–11
can be explained by the injected charges stored in nanocrystals or at the interface of the nanocrystals. The hysteresis is clockwise, indicating net negative charging, which should be due to electron trapping in the Ag nanodots. The large work function of Ag (about 4.3 eV) will create an asymmetrical barrier between the Si substrate and the storage nanodots, i.e., a small barrier for writing and a large barrier for retention [10], which gives much freedom for device optimization. After electrons are stored, at a chosen reading voltage (0.75 V, the flat-band voltage in our experiment) the trapped electrons have a probability to tunnel back to the Si substrate, which would cause a gradual shift in capacitance [14]. Time dependent capacitance measurements were performed to study the charge-retention characteristics of the Ag nanodots. If we defined the time for the loss of 10% of the stored electron as the retention time, the retention time of our device was found to exceed 8 h. The asymmetrical barrier between the Ag nanodots and the substrate (a lower barrier for writing and a higher barrier for retention) is of great advantage for the long retention time, as compared to semiconductor nanocrystals. In summary, we have demonstrated a synthesis method, which is capable of synthesis a monolayer of metal (Ag in this letter) nanodots on/embedded in Al2O3/Si substrate. Ag nanodots with the average diameter of 7 nm and the density of 7 1011 cm2 can be obtained. MIS structures using Al2O3 as gate dielectric embedded with Ag nanodots are fabricated. Clear electron storage effects are observed through the C–V and G–V measurements. A long charge-retention time is obtained due to the asymmetrical barrier between the Ag nanodots and the substrate. Our results demonstrate the feasibility of applying Ag nanodots for nanocrystals floating-gate memory device.
11
Acknowledgements The authors acknowledge the support from the Special Funds for Major State Basic Research Project No. G2001CB3095 of China and the National Natural Science Foundation of China through Grant Nos. 69925410, 60201004 and also the support from Shanghai Nanotechnology Promotion Center (No.0252nm084 and No.0359nm004).
References [1] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E.F. Crabbe, K. Chan, Appl. Phys. Lett. 68 (1996) 1377. [2] S. Tiwari, F. Rana, K. Chan, L. Shi, H. Hanafi, Appl. Phys. Lett. 69 (1996) 1232. [3] M.L. Ostraat, J.W. De Blauwe, M.L. Green, L.D. Bell, M.L. Brongersma, J. Casperson, R.C. Flagan, H.A. Atwater, Appl. Phys. Lett. 79 (2001) 433. [4] W.K. Choi, W.K. Chim, C.L. Heng, L.W. Teo, D.A. Antoniadis, E.A. Fitzgerald, Appl. Phys. Lett. 80 (2002) 2014. [5] 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. [6] D.N. Kouvatsos, V. Loannou-sougleridis, G. Nassiopoulou, Appl. Phys. Lett. 82 (2003) 397. [7] G.B. Assayag, C. Bonafos, M. Carrada, A. Claverie, Appl. Phys. Lett. 82 (2003) 200. [8] E.P. Gusev, M. Copel, E. Cartier, I.J.R. Baumvol, C. Krug, M.A. Gribelyuk, Appl. Phys. Lett. 76 (2000) 176. [9] M. Kundu, N. Miyata, M. Ichikawa, J. Appl. Phys. 93 (2003) 1498. [10] Z. Liu, C. Lee, V. Narayanan, G. Pei, E.C. Kan, IEEE Trans. Electr. Devices 49 (2002) 1606. [11] Q. Wan, N.L. Zhang, X.Y. Xie, T.H. Wang, C.L. Lin, Appl. Surf. Sci. 191 (2002) 171. [12] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [13] W. Ostwald, Z. Phys. Chem. (Leipzig) 34 (1900) 495. [14] S. Huang, S. Banerjee, R.T. Tung, S. Oda, J. Appl. Phys. 93 (2003) 576.