The role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 μm was investigated. As the dot size was increased, more Er ions were located near one dot due to its large surface area and more Er ions interacted with other ones. This Er-Er interaction caused a weak photoluminescence intensity, despite the increase in the effective excitation cross section. The critical dot size needed to take advantage of the positive effect on Er luminescence is considered to be about 2.0 nm, below which a small dot is very effective in the efficient luminescence of Er. © 2005 The Electrochemical Society. All rights reserved

Cho, KS

Kim, BH

Kim, KH

Kim, TY

Lee, JK

Nastasi, M

Park, NM

Park, SJ

Shin, JH

Sung, GY

The role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 μm was investigated. As the dot size was increased, more Er ions were located near one dot due to its large surface area and more Er ions interacted with other ones. This Er-Er interaction caused a weak photoluminescence intensity, despite the increase in the effective excitation cross section. The critical dot size needed to take advantage of the positive effect on Er luminescence is considered to be about 2.0 nm, below which a small dot is very effective in the efficient luminescence of Er

Park, Nae-Man

Kim, Tae-Youb

Kim, Kyung-Hyun

Sung, Gun Yong

Kim, Baek-Hyun

Park, Seong-Ju

Cho, Kwan Sik

Shin, Jung H.

Lee, Jung-Kun

Nastasi, Michael

Name not available

Effect of Amorphous Si Quantum-Dot Size on 1.54 μm Luminescence of Er

D-Scholarship@Pitt

Journal of The Electrochemical Society, 152 ~6! G445-G447 ~2005! G445DownlEffect of Amorphous Si Quantum-Dot Size on 1.54 mmLuminescence of ErNae-Man Park,a,z Tae-Youb Kim,a Kyung-Hyun Kim,a Gun Yong Sung,aBaek-Hyun Kim,b Seong-Ju Park,b Kwan Sik Cho,c Jung H. Shin,cJung-Kun Lee,d and Michael NastasidaBasic Research Laboratory, Electronics and Telecommunications Research Institute,Daejeon 305-350, KoreabDepartment of Materials Science and Engineering, Kwangju Institute of Science and Technology,Kwangju 500-712, KoreacDepartment of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, KoreadMaterials Science and Technology Division, Los Alamos National Laboratory, Los Alamos,New Mexico 87545, USAThe role of the size of amorphous silicon quantum dots in the Er luminescence at 1.54 mm was investigated. As the dot size wasincreased, more Er ions were located near one dot due to its large surface area and more Er ions interacted with other ones. ThisEr-Er interaction caused a weak photoluminescence intensity, despite the increase in the effective excitation cross section. Thecritical dot size needed to take advantage of the positive effect on Er luminescence is considered to be about 2.0 nm, below whicha small dot is very effective in the efficient luminescence of Er.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1901662# All rights reserved.Manuscript submitted August 30, 2004; revised manuscript received December 14, 2004. Available electronically April 22, 2005.0013-4651/2005/152~6!/G445/3/$7.00 © The Electrochemical Society, Inc.Er-doped silicon has attracted much interest because of its prom-ising future in the development of light-emitting diodes and lasersoperating at a wavelength of 1.54 mm, which coincides with theabsorption minimum of optical fibers.1,2 However, the intensity ofthe photoluminescence ~PL! of Er in this matrix is very weak atroom temperature. Attention is currently focused on Er-doped Sinanocrystals in SiOx which hold some promise for efficiently gen-erating light emission, as it was demonstrated that Si nanocrystals inthe presence of Er act as efficient sensitizers for Er ions.3-6 Amor-phous Si quantum dots ~a-Si QDs! have been fabricated previouslyand their role as an active layer in visible light-emitting diodes dem-onstrated, which stimulated interest in the control of dot size in asmall dimension compared to nanocrystals.7-9 Theoretical calcula-tions also showed that the radiative recombination rate for an a-SiQD is higher by two to three orders of magnitude than that for acrystalline Si QD,10 indicating that better performance can be ob-tained for a 1.54 mm light source when it is fabricated by using anEr-doped a-Si QD.In a recent report, the density effect of a very small a-Si clusteron Er PL in Si-rich SiO2 was investigated, where a high density ofa-Si clusters enhanced the PL efficiency compared to Sinanocrystals.11 Here, we report on the effect of the size of a-Si QDswith a mean size smaller than 2.5 nm on Er luminescence, which isanother effect for enhancing the PL efficiency. Large a-Si QDs ex-cite more Er ions than small ones because of the large surface areaof the dot. However, the luminescence decay time is much shorterand the Er PL intensity is very weak in a large-dot sample.50 nm thick silicon nitride films containing a-Si QDs weregrown on Si substrates by plasma-enhanced chemical vapor deposi-tion to obtain various dot sizes.8 Er+ ions were then implanted withan ion dose of 1 3 1021/cm3 into the silicon nitride films. The pro-file of implanted Er ions was monitored by Rutherford backscatter-ing spectroscopy. Finally, the samples were annealed at 900°C for0.5 h to reduce the residual defects left by the implantation process.The PL of the Er ions in the annealed films was measured using anAr laser. The samples were classified into three groups, referred toas large-dot, medium-dot, and small-dot samples in accordance withdot sizes ~diameters! of 2.5, 1.8, and 1.4 nm, respectively. Becausethe standard deviation was changed from 0.4 to 0.1 nm as the dotsize was decreased from 2.0 to 1.4 nm, it was clear that the sizes of1.8 and 1.4 nm were different. The change of dot size after the Erz E-mail: nmpark@etri.re.kr address. Redistribution subject to ECS terms130.49.166.8oaded on 2014-11-19 to IP ion implantation and the thermal annealing was confirmed by PLmeasurement, which showed the same peak position before and aftereach process. Therefore, the ion implantation and the thermal an-nealing at 900°C for 30 min are considered not to influence the dotsize.Figure 1 shows the PL spectra as a function of the dot size atroom temperature. The spectra show the luminescence of a-Si QDsin a visible range and a sharp peak at around 1.54 mm, which is acharacteristic transition between 4I13/2 and4I15/2 manifolds in Er3+ions. The emission peak position of a-Si QDs was controlled by thedot size due to a quantum size effect.7 For example, the peaks at640, 540, and 470 nm correspond to dot sizes of 2.5, 1.8, and1.4 nm, respectively. The small-dot sample shows a nearly fourtimes higher Er PL intensity than the large-dot sample. The lumi-nescence of Er ions is sensitive to energy transfer between Si dotsand Er ions; thus, these data show that a small dot is very effectivein Er luminescence and, as a result, enhances the luminescence ef-ficiency. To clearly identify the indirect excitation of Er ions byenergy transfer from a-Si QDs, the PL peak intensity of Er ions wasmeasured at different pump wavelengths. The inset in Fig. 1 showsthe PL intensity of Er ions at 1.54 mm as a function of the excitationwavelength at a fixed pump power of 10 mW. The absence of anabsorption resonance peak in the Er excitation spectrum indicatesthat Er ions are excited indirectly via an energy transfer from a-SiQDs to Er ions. The Er luminescence intensity decreases as theexcitation wavelength increases for medium- and small-dot samples,but remains nearly the same for a large-dot sample, which indicatesthat an alternate route exists to excite Er ions in addition to theenergy transfer from a-Si QDs. This is now discussed.Figure 2 shows the power dependency of Er PL vs. dot size. ThePL intensity is nearly proportional to the square root of the pumppower. The sublinear power dependence in Er:Si can be rationalizedas being due to the saturation of excitation of active Er ions,12 wherethe saturation power is dependent on the PL decay time; that is, thehigh saturation power is indicative of a short decay time. However,the saturation power increases with increasing decay time in oursamples when the dot size is decreased.Table I summarizes the decay time stdecayd, rise time stond, andthe effective excitation cross section ssd of Er PL for different dotsize samples at room temperature. The decay plots for all samplesshowed an exponential function expressed by Istd = I0 expf−st/tdg,where Istd and I0 are the intensity as a function of time, t and t= 0, respectively. t is the decay time. The decrease in tdecay valuewith increasing dot size indicates that the nonradiative process is) unless CC License in place (see abstract).  ecsdl.org/site/terms_use of use (see G446 Journal of The Electrochemical Society, 152 ~6! G445-G447 ~2005!Downllikely to be enhanced as the dot size increases. However, the effec-tive excitation cross section increases with increasing dot size. Theeffective excitation cross section can be obtained by measuring thePL rise time stond as a function of photon density, which is given by1/ton = sf + 1/tdecay, where f is the photon flux. Although the Erion concentration is the same for all samples, s is larger in a large-dot sample than in a small-dot sample. A previous study showed thatthe density of a-Si QD was increased about eightfold from 13 1019 to 8 3 1019 cm−3 when the dot size was decreased from2.0 to 1.4 nm.8 Considering the increases in the dot density and thetotal volume fraction of dots, the enhanced Er PL intensity withdecreasing dot size is reasonable because the luminescent Er ionsexcited by dots are increased. Therefore, it is expected that s mustincrease with decreasing dot size, but it follows an opposite trend,which cannot be explained by the dot density.The increase in s value with increasing dot size can be explainedby the fact that the absorption coefficient sad of the film increaseswith increasing dot size because s is proportional to a/nEr. In real-ity, a is increased about threefold with increasing dot size from1.4 to 2.5 nm. However, the s value was increased about tenfold asthe dot size was increased, which means that nEr should be de-Figure 2. PL intensity of Er ions as function of excitation power for variousdot size samples, measured at 25 K using 488 nm pump light.Figure 1. PL spectra as function of size of a-Si QD at room temperature. PLspectra of various sized a-Si QDs correspond to dot sizes of 2.5, 1.8, and1.4 nm, respectively. Inset shows PL intensity of Er ions as function ofexcitation wavelength for the various dot size samples at 25 K. address. Redistribution subject to ECS terms130.49.166.8oaded on 2014-11-19 to IP creased about 0.33 times with increasing dot size. In this case, thesaturation power can be decreased due to the decrease in nEr. Thedecrease of tdecay can also be understood by considering the increasein the refractive index with increasing dot size.13 Because the Er PLintensity is linearly proportional to nEr in a relation of IPL, nEr/trad, where trad is the radiative lifetime14 which is correlatedto the decay time ~1/tdecay = 1/trad + 1/tnr in which tnr is the non-radiative lifetime!, the decrease in Er PL intensity with increasingdot size results in a similar trad for all samples. However, the low-temperature PL decay measurement showed that tdecay at 25 K wasalmost the same as that at room temperature within 0.2 ms for allsamples, which means that the trad values are different from eachother. Therefore, another effect must be considered to clearly ex-plain the trend of Er PL intensity with varying the dot size in addi-tion to the variation of the absorption coefficient.Recently, luminescent Er ions were observed to be located nearlyat the surface of the nanoclusters.4 Jhe et al.15 also observed that thecharacteristic carrier-Er interaction distance is 0.5 nm, over whichthe interaction between carriers in an a-Si well and Er ions becomesvery weak. This is considered reasonable in our case. The surfacearea of a single dot is increased as the dot size is increased. A largesurface area of a single dot is considered to be very effective insensitizing Er ions because more Er ions can be located near one dotand interact with the dot. However, an excited Er ion can alsoreadily interact with other Er ions in the presence of one dot becausethe luminescent Er ions are close to each other. This constitutes thenegative effect in terms of luminescence efficiency. In Er-doped Sinanocrystals,16,17 the effective excitation cross section was increaseddue to the interaction of close Er pairs with increasing the Er contentin the film and the decay time was simultaneously decreased. In ourstudy, the same mechanism may be responsible for the correlationbetween the effective excitation cross section and PL intensity ~ordecay time! in various dot sizes, although the Er content was thesame for all samples. As the dot size increased, the number of opti-cally active Er ions near one a-Si QD increased, and the close spacebetween Er ions allows for energy exchange between neighboringions, which is the well-known concentration quenching effect. Thiseffect caused the increase in the effective excitation cross section,the decrease in the saturation power with increasing dot size, and thepump wavelength-independent PL intensity in a large-dot samplebecause Er ions are additionally excited due to resonant excitationby other Er ions. However, Er-Er interactions can be easily coupledto the nonradiative quenching sites and, thus, the PL decay time isdecreased and the PL intensity becomes weak. This effect was notobserved in the medium- and small-dot samples. If this explanationis correct, a large-dot sample must show an efficient PL at a small Erdose, compared to a small-dot sample. In reality, PL was observed ina large-dot sample implanted with an Er dose of 1 3 1019/cm3,whereas no PL was observed in the medium- and small-dot samples,as shown in Fig. 3. Therefore, these data suggest that the maximumdot size needed to take advantage of the positive effect of a-Si QDson Er luminescence without being affected by quenching phenom-ena due to Er-Er interactions is about 2.0 nm. In this study, thenitride matrix can play a role in exciting Er ions, but is considered tobe the same in all samples because the matrix compositions aresimilar in all samples, which was confirmed by the SiuN bondTable I. Rise time ton and decay time tdecay at 200 mW of488 nm pump light. Effective excitation cross section s wasobtained by ton as function of photon density f from 3 to 7ˆ 1018 s−1 cm−2 at room temperature with various dot sizes.Dot size ~nm! ton ~ms! tdecay ~ms! s s10−17 cm2dLarge ~2.5! 0.6 1.6 29Medium ~1.8! 1.6 2.8 4.2Small ~1.4! 1.8 3.0 2.7) unless CC License in place (see abstract).  ecsdl.org/site/terms_use of use (see G447Journal of The Electrochemical Society, 152 ~6! G445-G447 ~2005!Downlposition in Fourier transform infrared spectra. Therefore, the matrixeffect was excluded in the explanation for the Er luminescenceproperties.ConclusionsIn summary, Er PL properties as a function of the size of the a-SiQD were investigated in a silicon nitride film. The effect of dot sizeon Er PL is associated with an increase in the close Er pair interac-tions, which are more prevalent in a large-dot sample because alarge-dot sample has a large surface area and more luminescent Erions are located near one dot. This indicates that dot size is veryimportant in the efficient luminescence of Er.Figure 3. PL spectra for various dot size samples with Er concentration of~a! 1021 cm−3 and ~b! 1019 cm−3 at room temperature. Solid, dashed, anddashed-dot lines indicate large-, medium-, and small-dot samples, respec-tively. address. Redistribution subject to ECS terms130.49.166.8oaded on 2014-11-19 to IP AcknowledgmentsThis work was supported by the Ministry of Information andCommunication in Korea. J.-K.L. and M.N. thank the technical staffof the Ion Beam Materials Laboratory, Los Alamos National Labo-ratory, for their assistance.The Electronic and Telecommunications Research Institute assisted inmeeting the publication costs of this article.References1. 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Effect of amorphous Si quantum-dot size on 1.54 μm luminescence of Er

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Effect of amorphous Si quantum-dot size on 1.54 μm luminescence of Er

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