We report a study on the influence of charge trapping on electroluminescence (EL) from Si nanocrystal (nc-Si) distributed throughout a 30 nm Si O2 thin film synthesized by Si+ implantation into an oxide film thermally grown on a p -type Si substrate. The electron and hole trapping in the nc-Si located near the indium tin oxide gate and the Si substrate, respectively, cause a reduction in the EL intensity. The reduced EL intensity can be recovered after the trapped charges are released. A partial recovery can be easily achieved by the application of a positive gate voltage or thermal annealing at hot temperatures (e.g., 120 °C) for a short duration. The present study highlights the impact of charging in the nc-Si on the light emission efficiency and its stability of nc-Si light-emitting devices. © 2007 American Institute of Physics.published_or_final_versio

Chen, TP

Ding, L

Fung, S

Li, ZX

Liu, Y

Ng, CY

Tan, MC

Wong, JI

Yang, M

Yu, SF

Yuen, C

Zhu, FR

Journal of Applied Physics

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We report a study on the influence of charge trapping on electroluminescence (EL) from Si nanocrystal (nc-Si) distributed throughout a 30 nm SiO2 thin film synthesized by Si+ implantation into an oxide film thermally grown on a p-type Si substrate. The electron and hole trapping in the nc-Si located near the indium tin oxide gate and the Si substrate, respectively, cause a reduction in the EL intensity. The reduced EL intensity can be recovered after the trapped charges are released. A partial recovery can be easily achieved by the application of a positive gate voltage or thermal annealing at hot temperatures (e.g., 120 °C) for a short duration. The present study highlights the impact of charging in the nc-Si on the light emission efficiency and its stability of nc-Si light-emitting devices.Published versio

Liu, Yang

Chen, Tupei

Ding, Liang

Yang, Ming

Wong, Jen It

Ng, Chi Yung

Yu, Siu Fung

Li, Zeng Xiang

Yuen, Chau

Zhu, Fu Rong

Fung, Stevenson Hon Yuen

Tan, M. C.

Digital Repository - Nanyang Technological University

Influence of charge trapping on electroluminescence from Si-nanocrystal light emitting structure

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Title Influence of charge trapping on electroluminescence from Si-nanocrystal light emitting structureAuthor(s) Liu, Y; Chen, TP; Ding, L; Yang, M; Wong, JI; Ng, CY; Yu, SF; Li,ZX; Yuen, C; Zhu, FR; Tan, MC; Fung, SCitation Journal Of Applied Physics, 2007, v. 101 n. 10Issued Date 2007URL http://hdl.handle.net/10722/57344Rights Creative Commons: Attribution 3.0 Hong Kong LicenseInfluence of charge trapping on electroluminescence from Si-nanocrystallight emitting structureY. Liu, T. P. Chen,a L. Ding, M. Yang, J. I. Wong, C. Y. Ng, S. F. Yu, Z. X. Li, andC. YuenSchool of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798F. R. Zhu and M. C. TanInstitute of Materials Research and Engineering, Singapore 117602S. FungDepartment of Physics, The University of Hong Kong, Hong KongReceived 30 October 2006; accepted 26 January 2007; published online 18 May 2007We report a study on the influence of charge trapping on electroluminescence EL from Sinanocrystal nc-Si distributed throughout a 30 nm SiO2 thin film synthesized by Si+ implantationinto an oxide film thermally grown on a p-type Si substrate. The electron and hole trapping in thenc-Si located near the indium tin oxide gate and the Si substrate, respectively, cause a reduction inthe EL intensity. The reduced EL intensity can be recovered after the trapped charges are released.A partial recovery can be easily achieved by the application of a positive gate voltage or thermalannealing at hot temperatures e.g., 120 °C for a short duration. The present study highlights theimpact of charging in the nc-Si on the light emission efficiency and its stability of nc-Silight-emitting devices. © 2007 American Institute of Physics. DOI: 10.1063/1.2713946I. INTRODUCTIONSince silicon is the core material for microelectronic de-vices, there is intensive interest in the integration of siliconoptical/optoelectronic devices for the emission, modulation,and detection on one chip based on the mainstream Si tech-nology. One of the main bottlenecks in working toward aphotonic integration compatible with the Si technology is thelack of an efficient Si-based light source. In recent years,Si-nanocrystal structures provide the possibility of Si-basedlight emitting devices.1–9 One of the promising methods tosynthesize the Si nanocrystals nc-Si embedded in SiO2 di-electric films for the light emitting structures is Si ion im-plantation into SiO2 thin films. Electroluminescence EL atroom temperature from such structures has been observed bydifferent researchers,10–12 which represents a very importantstep towards the development of Si-based optoelectronics.However, there are still many problems and difficulties suchas the low efficiency and efficiency degradation of the EL. Inthis work we will show that the efficiency degradation is dueto the charge trapping in the Si nanocrystals. We have ob-served that the EL intensity can be modulated by thecharging/discharging in the nc-Si, i.e., the EL intensity isreduced by the charge trapping in the nc-Si while it is recov-ered by the release of the trapped charges from the nc-Si.II. EXPERIMENTALSiO2 films with the thickness of 30 nm were thermallygrown on 100 oriented, p-type Si substrates at 950 °C. Si+ions were implanted into the oxide films at 8 keV with adose of 21015 cm−2. After the ion implantation, thermalannealing was carried out at 1000 °C for 100 min to inducethe formation of nc-Si. Light-emitting structures were fabri-cated using a 130 nm indium tin oxide ITO layer and a500 nm Al layer to form the top and bottom electrodes, re-spectively. For the capacitance-voltage C-V measurement,the structure with aluminum top electrode instead of the ITOelectrode was also prepared. The area of the top electrodesvaries from 7.9103 to 4.5106 m2. The as-depositedITO layer has a sheet resistance of 25  / sq, a surface rms1.0 nm, and an average transmittance of above 85% overthe visible wavelengths of 400–800 nm. The stopping andrange of ions in matter SRIM simulation shows that theimplanted Si ions distribute throughout the entire 30 nm ox-ide with the peak concentration of excess Si atoms located ataround 15 nm underneath the oxide surface, as illustrated inFig. 1a. Figure 1b shows the transmission electron mi-croscopy TEM image of nc-Si with the sizes of 3–4 nmembedded in the SiO2 matrix. Photoluminescence PL mea-surement was performed at an excitation wavelength of325 nm, but no PL was observed in the as-fabricated sample.The C-V measurement was performed with a HP 4284A LCRmeter, while the current measurement was carried out with aKeithley 4200 semiconductor characterization system.III. RESULT AND DISCUSSIONFigure 2a shows the experimental setup of the EL mea-surement. Under a negative bias applied to the ITO gate andwith the substrate grounded, electrons and holes are injectedfrom the ITO gate and the Si substrate, respectively, andradiative recombination of the injected electrons and holesleads to light emission. At the same time, some of the in-jected charges are trapped in the nc-Si as shown in Fig. 2a,affecting the carrier injections and thus the EL. The EL fromaAuthor to whom correspondence should be addressed; electronic mail:echentp@ntu.edu.sgJOURNAL OF APPLIED PHYSICS 101, 104306 20070021-8979/2007/10110/104306/4/$23.00 © 2007 American Institute of Physics101, 104306-1Downloaded 13 Oct 2008 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspthe light-emitting structure can be observed with naked eyesin a dark environment. Figure 2b shows EL spectra mea-sured under the voltages of −14, −16, and −18 V, respec-tively. The EL exhibits a broad spectrum peaked at 500 nm2.5 eV. The EL intensity increases with the injection cur-rent. No photoluminescence PL from the same Si+ im-planted SiO2 film was observed, which could be due to thefact that the nanocrystals were still not fully stabilized toexhibit PL after the annealing at the slightly lower tempera-ture i.e., 1000 °C. Indeed, it was reported that the anneal-ing at 1100 °C shows the PL related to the formation of Sinanocrystal.13 On the other hand, as the energy 2.5 eV ofthe EL peak is significantly larger than the band gap1.8–2.1 eV of the nc-Si with a size of 3–4 nm,14 itseems that the EL cannot be simply attributed to the quantumconfinement effect. It is believed that the defects in theSi+-implanted SiO2 film, which serve as the luminescentcenters,10,15–17 play an important role in the light emission.As EL intensity is determined by the numbers of theinjected electrons and holes available for the radiative re-combination, charge trapping in the nc-Si,18 which affects thecarrier injection, should have an impact on the light emis-sion. This has been confirmed in our experiment. As can beobserved in Fig. 3a, the application of −25 V for 1 s leadsto a drastic reduction in the light emission intensity as com-pared to the virgin case, which is attributed to the chargetrapping in nc-Si. The large negative flatband voltage shiftVFB=−1.7 V after the application of −25 V for 1 s in theC-V characteristic shown in Fig. 3b indicates a largeamount of hole trapping in the nc-Si located near the inter-face of SiO2/Si substrate. The equivalent areal density Qof trapped holes corresponding to the negative flatband volt-age shift can be estimated withQ = − VFBCox, 1where Cox =0 SiO2 /d, where 0 is permittivity in vacuum,SiO2 is the dielectric constant of SiO2, and the oxide thick-ness d=30 nm is the oxide capacitance per unit area. For theapplication of −25 V for 1 s, Q is 1.21012 holes/cm2.The hole trapping is due to the hole injection from the sub-strate under the negative gate bias. The injected holes can betrapped in any nc-Si distributed in the oxide, but most of thehole trapping should occur in the interface region as the in-jected holes can be easily trapped in the nc-Si located nearthe injection interface. The hole trapping will lead to a re-duction in the electric field for the hole injection from thesubstrate. Similarly, some of the injected electrons from theITO gate may also be trapped in the nc-Si with most of theelectron trapping occurring near the gate, and thus the elec-tric field for the electron injection from the ITO gate is alsoreduced. Note that the electron trapping near the gate cannotbe readily revealed from the C-V measurement because thetrapped electrons are far away from the SiO2/Si substrateFIG. 1. a Depth distribution of the excess Si concentration in the 30 nmSiO2 layer obtained from SRIM simulation, and b TEM image of the nc-Siembedded in SiO2 matrix. The implantation energy is 8 keV.FIG. 2. a Experimental setup for the EL measurement and illustration ofcharging in the nc-Si, and b EL spectra under various measurementvoltages.104306-2 Liu et al. J. Appl. Phys. 101, 104306 2007Downloaded 13 Oct 2008 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspinterface. With reductions in the electric fields for both holeand electron injections, the injection current decreases. In-deed, as shown in Fig. 3c, the current at the voltage of−14 V note that the EL measurements shown in Fig. 3awere carried out at −14 V for the virgin case is much largerthan that after the application of −25 V for 1 s. With thereduction of the injection current, the number of injectedholes and electrons for the radiative recombination de-creases, and thus the EL intensity decreases. It should bepointed out that the charge trapping occurs not only at thehigher voltage of −25 V but also at the relatively low voltageof −14 V. As shown in Fig. 3c, the current measured at−14 V for both the virgin case and after the application of−25 V for 1 s decreases with the biasing time, indicating theaccumulation of charges trapped in the nc-Si. This meansthat the EL intensity would degrade during the EL operation.On the other hand, as can be seen in Fig. 3a, the re-duced EL intensity is partially recovered by the applicationof +25 V for 5 s. The recovery is due to the release of someof the charges trapped in the nc-Si under the influence of thepositive gate voltage. As shown in Fig. 3b, the applicationof +25 V for 5 s leads to a partial recovery in the flatbandvoltage shift, i.e., the VFB is partially recovered from−1.7 V for the case of application of −25 V for 1 s to −1 Vnow. The reduction in the flatband voltage shift indicates thatsome of the trapped holes have been released by the appli-cation of the positive voltage 41% of the trapped holes havebeen released. Under a positive gate voltage, some of thetrapped holes can tunnel out from the nc-Si or recombinewith the electrons injected from the substrate. After the re-lease of some of the trapped holes, for an EL or currentmeasurement at a negative gate voltage, the electric field atthe SiO2/Si substrate interface increases, and thus the holeinjection from the substrate increases. On the other hand,some of the trapped electrons near the gate can also be re-leased under the influence of a positive gate voltage, andthen the electron injection from the gate also increases in theEL or current measurement. Therefore, with the release ofsome of the trapped holes and electrons, a partial recovery inthe current is achieved as shown in Fig. 3c. This explainsthe above-mentioned partial recovery in the EL intensity. Thetrapped charges can also be released by a hot temperatureannealing for a short period. Figure 4 shows the effect of theannealing at 120 °C for 40 s. As can be seen in this figure,the annealing leads to a partial recovery in the EL intensity,the flatband voltage shift, and the injection current, and thesituation is very similar to that of the application of the posi-tive bias discussed above. The relatively low temperatureand short duration of the hot-temperature annealing indicatesthat some of the trapped charges are easily released whenthey gain the thermal energy. This could suggest that onlythose nc-Si located near the Si substrate and the ITO gate areinvolved in the process as the charges trapped in them caneasily tunnel out to the gate or escape to the substrate at anelevated temperature.Also it should be pointed out that with the accumulationof trapped charges i.e., the trapped electrons near the gateand the trapped holes near the substrate under a negativebias, the injection current would degrade with time. Indeed,as can be seen in Fig. 3c, the injection current at −14 Vdecreases with time. The decay of injection current is alsoobserved on the structures with Al gate, as shown in Fig. 5.This indicates that the current degradation is not related tothe ITO gate, and it further supports the argument that thecharge trapping in the nc-Si leads to decrease in the injectioncurrent.18 The current decay means degradation in the ELintensity as a result of the accumulation of trapped charges inthe nc-Si. As discussed above, the trapped charges could bereleased under a positive gate bias. This suggests that bipolarpulses i.e., the pulses change between negative and positiveFIG. 3. Influence of trapping and detrapping of charges in nc-Si due to theapplication of −25 V for 1 s and +25 V for 5 s, respectively, on the EL a,the C-V characteristic b, and the injection current c. Both the EL and thetime-domain current measurements were carried out at −14 V.104306-3 Liu et al. J. Appl. Phys. 101, 104306 2007Downloaded 13 Oct 2008 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jspvoltages alternately could eliminate the charging effect andthus maintain the EL efficiency as reported in Ref. 19.IV. CONCLUSIONIn conclusion, the influence of charge trapping on the ELemission from ITO/Si+ implanted SiO2 layer/Si structure hasbeen studied. It is shown that the charge trapping in the nc-Sinear the gate and the substrate can cause a large reduction inthe EL intensity due to the decrease of the electric fields forboth the electron injection from the gate and the hole injec-tion from the substrate. However, the reduced EL intensitycan be recovered after the trapped charges are released. Apartial recovery can be easily achieved by the application ofa positive gate voltage or thermal annealing at hot tempera-tures e.g., 120 °C for a short duration. The results suggestthat bipolar pulses i.e., the pulses change between negativeand positive voltages alternately could eliminate the charg-ing effect and thus maintain the EL efficiency.ACKNOWLEDGMENTThis work has been financially supported by the Aca-demic Research Fund of the Ministry of Education, Sin-gapore, under Project No. ARC 1/04 and by the SingaporeMillennium Fund.1A. Irrera et al., Appl. Phys. Lett. 81, 1866 2002.2L. Rebohle, J. von Borany, D. Borchert, H. Fröb, T. Gebel, M. Helm, W.Möller, and W. Skorupa, Electrochem. Solid-State Lett. 4, G57 2001.3G. Franzò et al., Appl. Phys. A: Mater. Sci. Process. 74, 1 2002.4A. Irrera et al., Opt. Mater. Amsterdam, Neth. 27, 1031 2005.5P. Photopoulos, A. G. Nassiopoulou, D. N. Kouvatsos, and A. Travlos,Mater. Sci. Eng., B 69–70, 345 2000.6P. Photopoulos and A. G. Nassiopouloua, Appl. Phys. Lett. 77, 18162000.7M. E. Castagna, S. Coffa, M. Monaco, A. Muscara, L. Caristia, S. Lorenti,and A. Messina, Mater. Sci. Eng., B 105, 83 2003.8M. E. Castagna, S. Cofa, M. Monaco, L. Caristia, A. Messina, R.Mangano, and C. Bongiorno, Physica E Amsterdam 16, 547 2003.9A. Irrera et al., Physica E Amsterdam 16, 395 2003.10D. Muller, P. Knapek, J. Faure, B. Prevot, J. J. Grob, B. H. Onerlage, andI. Pelant, Nucl. Instrum. Methods Phys. Res. B 148, 997 1999.11L. Rebohle, J. von Borany, H. Fröb, and W. Skorupa, Appl. Phys. B:Lasers Opt. 71, 131 2000.12H.-Z. Song, X.-M. Bao, N.-S. Li, and J.-Y. Zhang, J. Appl. Phys. 82, 40281997.13T. Shimizu-Iwayama, S. Nakao, and K. Saitoh, Appl. Phys. Lett. 65, 18141994.14L. Ding, T. P. Chen, Y. Liu, C. Y. Ng, and S. Fung, Phys. Rev. B 72,125419 2005.15L.-S. Liao, X.-M. Bao, N.-S. Li, X.-Q. Zheng, and N.-B. Min, Solid StateCommun. 97, 1039 1996.16T. D. Shen et al., Phys. Rev. B 55, 7615 1997.17S. M. Prokes, W. E. Carlos, S. Veprek, and Ch. Ossadnik, Phys. Rev. B 58,15632 1998.18Y. Liu, T. P. Chen, C. Y. Ng, M. S. Tse, S. Fung, Y. C. Liu, S. Li, and P.Zhao, Electrochem. Solid-State Lett. 7, G134 2004.19R. J. Walters, G. I. Bourianoff, and H. A. Twater, Nat. Mater. 4, 1432005.FIG. 4. Influence of trapping and detrapping of charges in nc-Si due to theapplication of −25 V for 1 s and the annealing at 120 °C for 40 s, respec-tively, on the EL a, the C-V characteristic b, and the injection current c.Both the EL and the time-domain current measurements were carried out at−14 V.FIG. 5. Degradation of the injection current measured at −14 V for thestructure with Al gate.104306-4 Liu et al. J. Appl. Phys. 101, 104306 2007Downloaded 13 Oct 2008 to 147.8.17.95. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

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Influence of charge trapping on electroluminescence from Si-nanocrystal light emitting structure

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