The authors report the photon-induced conduction modulation in Si O2 thin films embedded with germanium nanocrystals (nc-Ge). The conduction of the oxide could be switched to a higher- or lower-conductance state by a ultraviolet (UV) illumination. The conduction modulation is caused by charging and discharging in the nc-Ge due to the UV illumination. If the charging process is dominant, the oxide conductance is reduced; however, if the discharging process is dominant, the oxide conductance is increased. As the conduction can be modulated by UV illumination, it could have potential applications in silicon-based optical memory devices. © 2007 American Institute of Physics.published_or_final_versio

Chen, TP

Ding, L

Fung, S

Liu, Y

Tan, MC

Trigg, AD

Tung, CH

Wong, JI

Yang, M

Yu, SF

Zhu, FR

Applied Physics Letters

HKU Scholars Hub

Title Photon-induced conduction modulation in SiO 2 thin filmsembedded with Ge nanocrystalsAuthor(s) Ding, L; Chen, TP; Yang, M; Wong, JI; Liu, Y; Yu, SF; Zhu, FR;Tan, MC; Fung, S; Tung, CH; Trigg, ADCitation Applied Physics Letters, 2007, v. 90 n. 10Issued Date 2007URL http://hdl.handle.net/10722/80507Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Photon-induced conduction modulation in SiO2 thin films embeddedwith Ge nanocrystalsL. Ding, T. P. Chen,a M. Yang, J. I. Wong, Y. Liu, and S. F. YuSchool of Electrical and Electronic Engineering, Nanyang Technological University,Singapore 639798, SingaporeF. R. Zhu and M. C. TanInstitute of Materials Research and Engineering, Singapore 117602, SingaporeS. FungDepartment of Physics, The University of Hong Kong, Hong KongC. H. Tung and A. D. TriggInstitute of Microelectronics, Singapore 117685, SingaporeReceived 22 December 2006; accepted 31 January 2007; published online 6 March 2007The authors report the photon-induced conduction modulation in SiO2 thin films embedded withgermanium nanocrystals nc-Ge. The conduction of the oxide could be switched to a higher- orlower-conductance state by a ultraviolet UV illumination. The conduction modulation is caused bycharging and discharging in the nc-Ge due to the UV illumination. If the charging process isdominant, the oxide conductance is reduced; however, if the discharging process is dominant, theoxide conductance is increased. As the conduction can be modulated by UV illumination, it couldhave potential applications in silicon-based optical memory devices. © 2007 American Institute ofPhysics. DOI: 10.1063/1.2711198Si and Ge nanocrystals embedded in SiO2 matrix havebeen demonstrated as the potential candidates for the fabri-cation of Si-based optoelectronic devices1,2 and nonvolatilememory devices3,4 that are compatible with the main streamcomplementary metal-oxide-semiconductor process. One ofthe promising techniques being used to synthesize suchstructures is the implantation of Si or Ge ions into SiO2 filmsfollowed by high temperature annealing. The memory effectof semiconductor nanocrystals embedded in SiO2 thin filmssynthesized with low energy ion implantation followed bysubsequent annealing has been demonstrated by several re-search groups.3,5–9 In these nanocrystals-based memory de-vices, the conventional polysilicon floating gate is replacedby isolated nanocrystals embedded in the gate oxide whichcan increase the retention time, scalability, and reliability. Inthis letter, we report a conduction modulation effect of SiO2films embedded with Ge nanocrystals nc-Ge caused by ul-traviolet UV illumination. Unlike the conventional metal-oxide-semiconductor MOS structures with nanocrystalsembedded in the gate oxide, we fabricated a MOS-like de-vice structure of which the metal gate was replaced by in-dium tin oxide ITO. In this study, the ITO film plays therole as a semitransparent electrode with which the light canpenetrate the gate electrode.A 30 nm SiO2 thin film was thermally grown in dryoxygen at 950 °C on p-type Si100 wafer. Ge ions with adose of 21015 cm−2 were then implanted into the SiO2 thinfilm at the energy of 6 keV. Thermal annealing was carriedout in N2 ambient at 800 °C for 60 min. Finally, a 130 nmITO thin film was deposited on the Ge+-implanted SiO2 thinfilm to form a semitransparent gate electrode with a diameterof 1.2 mm. Details of the ITO deposition and characteriza-tion can be found in previous studies.10,11 The device struc-ture is presented in Fig. 1a. As revealed by the simulationof stopping and range of ion in matter SRIM shown inFig. 1b, the implanted Ge distributes from the oxide surfaceto a depth of 20 nm. The formation of nc-Ge was con-aElectronic mail: echentp@ntu.edu.sgFIG. 1. Color online a Schematic illustration of the device structure. bDistribution of nc-Ge in the gate oxide obtained from SRIM simulation. cCross-sectional TEM image of nc-Ge embedded in SiO2.APPLIED PHYSICS LETTERS 90, 103102 20070003-6951/2007/9010/103102/3/$23.00 © 2007 American Institute of Physics90, 103102-1Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsfirmed by cross-sectional transmission electron microscopyTEM measurement, as shown in Fig. 1c. The current-voltage I-V and time-domain current measurements wereconducted with a Keithley-4200 semiconductor characteriza-tion system at room temperature. The UV illumination wascarried out with an Oriel-66011 arc lamp together with anOriel-77250 monochromator. The wavelength used for theUV illumination is 365 nm.Figure 2 shows the I-V characteristics of the structurewith nc-Ge distributed in the gate oxide before UV illumina-tion. Note that the maximum voltage of the I-V measurementis 4 V which is low enough such that no charging/discharging effect occurs due to the electrical measurementitself. As can be seen in Fig. 2, the repeated measurementsdid not cause a significant change in the I-V characteristic.This indicates that no significant charging or discharging innc-Ge occurs during the I-V measurement and the conduc-tion of the gate oxide was not affected by the measurementitself. However, after an exposure to UV illumination, theI-V characteristic is found to change drastically. Figure 3shows the comparison between the I-V characteristics beforeand after the UV illumination for 5 s. As can be seen in thisfigure, the current is reduced by more than 50% after the UVillumination. The reduction in the current indicates a largeincrease in the dc resistance or decrease in the conductanceof the gate oxide.The increase in the dc resistance can be explained interms of the breaking of some tunneling paths due to charg-ing in some nc-Ge caused by UV illumination. Electron tun-neling can take place between adjacent uncharged nanocrys-tals, and a large number of such nanocrystals in the oxide canform many conductive tunneling paths which significantlyincrease the conductance of the gate oxide, as shown inFig. 4a. However, if some of the nc-Ge is charged, thetunneling paths related to the charged nc-Ge will be blockeddue to the Coulomb blockade effect, as shown in Fig. 4b.As a result of the charging in the nc-Ge, the conductance ofthe oxide decreases. If the sample is exposed to UV light,some of the electrons generated by the UV illuminationcould be trapped in the nc-Ge, leading to a reduction in theoxide conductance. At the same time, the UV illuminationcould cause the release of the trapped electrons from thenc-Ge also, resulting in an increase in the oxide conductance.Charging and discharging in the nc-Ge due to the UV illu-mination are two competing processes which occur simulta-neously. If the charging process is dominant, the oxide con-ductance is reduced; however, if the discharging process isdominant, the oxide conductance is increased. The two situ-ations have been observed in our experiment. As shown inFig. 3 an UV illumination for 5 s leads to a large reduction inthe oxide conductance; however, a further UV illuminationfor 200 s results in an increase a partial recovery in theconductance as compared to the conductance of the 5 s illu-mination, as shown in Fig. 5a. On the other hand, as shownin Fig. 5b, the oxide conductance can recover almost to thelevel of the virgin situation after a thermal annealing at150 °C for 15 min in addition to the UV illumination for200 s. This indicates that almost all the charges trapped inthe nc-Ge due to the first UV illumination can be releasedafter the second UV illumination and the thermal annealing.As mentioned above, the charging and discharging innc-Ge could occur simultaneously under UV illumination.This suggests that the conduction of the gate oxide can bemodulated by UV illumination in a time-domain measure-ment at a constant measurement voltage. This is confirmed inour experiment. Figures 6a and 6b show the time-domainFIG. 2. Repeated current-voltage I-V measurements without UVillumination.FIG. 3. I-V characteristics before and after UV illumination for 5 s.FIG. 4. Color online Proposed model of the effect of charging and dis-charging in nc-Ge on the current conduction of the gate oxide. a Conduc-tive tunneling paths formed by uncharged nc-Ge and b blocked paths dueto charging in nc-Ge.103102-2 Ding et al. Appl. Phys. Lett. 90, 103102 2007Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsdc resistance measurement at 4 V without and under UVillumination, respectively. As can be seen from Fig. 6a, nosignificant resistance change is observed, indicating that nosignificant charging and discharging in nc-Ge occur duringthe measurement without UV illumination. However, the re-sistance is modulated under the UV illumination, as shown inFig. 6b. As can be seen from this figure, after the first UVillumination for 4 s, the resistance is switched from a low-resistance state state 1, which corresponds to the situationof no charge trapping in the nc-Ge, to a high-resistance statestate 2 as a result of charging in the nc-Ge due to the UVillumination. State 2 is maintained for about 120 s, and thenthe resistance is switched to a new state state 3 which has aresistance much lower than that of state 2 but slightly higherthan that of state 1. This indicates that not all of the trappedcharges associated with state 2 have been released and thusonly a partial recovery of the conduction is achieved. Afterstaying at state 3 for about 370 s, the resistance is switchedback to state 2 as the charging process is dominant again. Ifthe time scale is extended, the switching between differentstates, which is a random event, can be observed continu-ously. However, it is generally observed that for a virginsample a short UV exposure leads to a switching to a high-resistance state i.e., the charging process is dominant whilea further exposure could cause a switching back to a low-resistance state i.e., the discharging process is dominantdepending on the exposure duration.In conclusion, we have observed UV-illumination-induced conduction modulation in SiO2 films embedded withnc-Ge synthesized by ion implantation. The conductionmodulation is due to the charging and discharging in thenc-Ge caused by UV illumination. The charging and dis-charging in the nc-Ge are two competing processes whichoccur simultaneously. If the charging process is dominant,the oxide conductance is reduced; however, if the discharg-ing process is dominant, the oxide conductance is increased.As the conduction can be modulated by UV illumination, itcould have potential applications in silicon-based opticalmemory devices.This work has been financially supported by the Ministryof Education, Singapore, under Project No. ARC 1/04.1R. J. Walters, G. I. Bourianoff, and H. A. Atwater, Nature London 4,143 2005.2J.-Y. Zhang, Y.-H. Ye, and X.-L. Tan, Appl. Phys. Lett. 74, 2459 1999.3C. Y. Ng, T. P. Chen, L. Ding, and S. Fung, IEEE Electron Device Lett.27, 231 2006.4K. Choi, W. K. Chim, C. L. Heng, L. W. Teo, V. Ho, V. Ng, D. A.Antoniadis, and E. A. Fitzgerald, Appl. Phys. Lett. 80, 2014 2002.5Y. Liu, T. P. Chen, M. S. Tse, H. C. Ho, and K. H. Lee, Electron. Lett. 39,1164 2003.6E. Kapetanakis, P. Normand, D. Tsoukalas, and K. Beltsios, Appl. Phys.Lett. 80, 2794 2002.7C. Y. Ng, T. P. Chen, M. Yang, J. B. Yang, L. Ding, C. M. Li, A. Du, andA. Trigg, IEEE Trans. Electron Devices 53, 663 2006.8C. Y. Ng, T. P. Chen, P. Zhao, L. Ding, Y. Liu, Ampere A. Tseng, and S.Fung, J. Appl. Phys. 99, 106105 2006.9C. J. Park, K. H. Cho, W.-C. Yang, H. Y. Cho, S.-H. Choi, R. G. Elliman,J. H. Han, and C. Kim, Appl. Phys. Lett. 88, 071916 2006.10K. Zhang, F. R. Zhu, C. H. A. Huan, and A. T. S. Wee, J. Appl. Phys. 86,974 1999.11J. Q. Hu, J. S. Pan, F. R. Zhu, and H. Gong, J. Appl. Phys. 95, 62732004.FIG. 6. Time-domain measurement of the dc resistance of the oxide: awithout UV illumination and b with UV illumination. The dc resistancewas measured at a gate voltage of 4 V.FIG. 5. a Partial recovery of the gate current after another UV illuminationfor 200 s from the situation of the UV illumination for 5 s. b Full recoveryof the gate current after an annealing at 150 °C for 15 min in addition to theUV illumination for 200 s.103102-3 Ding et al. Appl. Phys. Lett. 90, 103102 2007Downloaded 19 Apr 2012 to 147.8.21.150. 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Photon-induced conduction modulation in SiO 2 thin films embedded with Ge nanocrystals

The authors report the photon-induced conduction modulation in SiO2 thin films embedded with germanium nanocrystals (nc-Ge). The conduction of the oxide could be switched to a higher- or lower-conductance state by a ultraviolet (UV) illumination. The conduction modulation is caused by charging and discharging in the nc-Ge due to the UV illumination. If the charging process is dominant, the oxide conductance is reduced; however, if the discharging process is dominant, the oxide conductance is increased. As the conduction can be modulated by UV illumination, it could have potential applications in silicon-based optical memory devices.Published versio

Tan, M. C.

Ding, Liang

Chen, Tupei

Yang, Ming

Wong, Jen It

Liu, Yang

Yu, Siu Fung

Zhu, Fu Rong

Fung, Stevenson Hon Yuen

Tung, Chih Hang

Trigg, Alastair David

Digital Repository - Nanyang Technological University

Photon-induced conduction modulation in SiO2 thin films embedded with Ge nanocrystals

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Photon-induced conduction modulation in SiO 2 thin films embedded with Ge nanocrystals

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