Photoluminescence from Er-doped silicon oxide microcavities

Optical Materials 28 (2006) 873–878 www.elsevier.com/locate/optmat

Photoluminescence from Er-doped silicon oxide microcavities A. Hryciw b

a,* ,

C. Blois a, A. Meldrum a, T. Clement b, R. DeCorby b, Quan Li

c

a Department of Physics, University of Alberta, Edmonton, AB, Canada T6G 2J1 Department of Electrical and Computer Engineering and TRLabs, University of Alberta, Edmonton, AB, Canada T6G 2V4 c Department of Physics, Science Centre North Block, The Chinese University of Hong Kong, Shatin, NT, Hong Kong

Available online 9 November 2005

Abstract Er-doped silicon oxide (SiO:Er) thin films with Er concentrations ranging from 0.04 to 2.5 at.% were deposited on SiO2 and Si substrates by co-evaporation of SiO and Er2O3 under high vacuum. Electron energy filtered imaging and elemental mapping confirm the presence of amorphous Si nanoclusters surrounded by a SiO2 matrix. Steady-state and time-resolved photoluminescence indicate that the 1.54 lm emission is highest for a 0.20-at.%-Er specimen annealed at 500 C in 95% N2 + 5% H2, yielding effective excitation cross-sections in the range of 1016 cm2 for 476 nm excitation. To narrow and tune the Er emission, we have incorporated SiO:Er into planar microcavities with metal mirrors. The low thermal processing temperatures permitted the demonstration of simple-to-fabricate optical microcavities with intensified and directional emission in the 1480–1610 nm range.  2005 Elsevier B.V. All rights reserved.

1. Introduction Much of the recent interest in rare-earth doped silicon nanocrystals (nc-Si) has been driven by their potential use in silicon-based integrated optoelectronics [1]. The intense 1.54 lm emission from the 4I13/2 ! 4I15/2 intra-4f transition of Er3+ has made erbium-doped nc-Si (nc-Si:Er) particularly interesting from an integrated optics standpoint, as this corresponds to the wavelength of minimum attenuation in conventional silica optical fibers. While Er-doped SiO2 suffers from ion clustering effects and low luminescence efficiency, co-doping with Si nanoparticles can increase the 1.54 lm luminescence efficiency by as much as two orders of magnitude [2]. Thus, Er-doped Si nanocomposites comprise a potentially attractive class of materials for application in optical waveguide amplifiers. Most recent studies of Er-doped nc-Si have relied on thermal processing on the order of 1000 C or more to induce phase separation in silicon oxides containing excess Si, precipitating it into nanocrystals surrounded by a SiO2 matrix e.g. Refs. [3–5]. While high-quality, well-passivated *

Corresponding author. Tel.: +1 780 492 5135. E-mail address: [email protected] (A. Hryciw).

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nanocrystals may thus be produced [6], the high annealing temperatures are incompatible with standard complementary metal oxide semiconductor (CMOS) fabrication processes [7]. This poses a significant barrier to the monolithic integration of nc-Si:Er-based photonics components with CMOS driving circuitry for optoelectronic applications. Low-temperature methods of producing Er-doped Si nanocomposites with comparable luminescent properties are therefore of practical interest. For example, erbiumdoped semi-insulating crystalline and amorphous Si (SIPOS) [8,9], and silicon monoxide [10] can exhibit intense 1.54 lm emission after annealing at temperatures on the order of 400–600 C. Relatively recently, the optical properties of such Er-doped silicon-rich oxides have received renewed interest e.g. Refs. [11–13]. In this study, we investigate the photoluminescence from Er-doped SiO produced via standard thin film deposition techniques. The microstructure of the SiO:Er, and its effect on the emission mechanism, is examined. We optimize the 1.54 lm luminescence with respect to annealing temperature and Er concentration. To further illustrate the value of this low-temperature-anneal, we have constructed planar Fabry–Perot microcavities with metal mirrors to spectrally narrow and tune the emission, using fabrication steps

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that should be compatible with the post-metal processing stages of CMOS production. The resonant wavelength of these cavities is tunable across the entire 1.54 lm Er3+ emission band simply by varying the SiO:Er layer thickness. 2. Experimental Five 150-nm-thick films of silicon oxide with varying Er doping concentrations were deposited under high vacuum on high-purity SiO2 substrates via co-evaporation of SiO and Er2O3 at a base pressure of 1.5 · 104 Pa. The silicon monoxide was thermally evaporated using a baffled box source, whereas electron-beam evaporation was used for the erbium oxide. The samples were subsequently annealed for 1 h in flowing N2, Ar, or forming gas (95% N2 + 5% H2), at temperatures ranging from 300 to 1000 C. Electron microprobe analysis (EMPA) was employed to obtain compositional analyses of the films, using SiO2 and ErPO4 as standards (Smithsonian), and a beam energy of 3 kV. The structural composition of the films was determined using transmission electron microscopy (TEM) and energy filtered imaging. Steady-state photoluminescence (PL) spectra were collected with a fiber-optic system and analysed with InGaAs and Si CCD spectrometers, using the 325 nm line of a HeCd laser or the 476 nm line of an argon ion laser for continuous wave excitation. The spectral response of the spectroscopy system was corrected by normalizing to a standard blackbody radiator. PL lifetime measurements were obtained using a thermoelectrically cooled amplified InGaAs photodiode connected to a digital storage oscilloscope. The 476 nm laser beam was chopped using an acousto-optic modulator at a frequency of 10 Hz, with 20 mW of power incident on the specimen during excitation over a circular spot 0.6 mm in radius. PL collected by optical fibers was passed through a 1550 nm band-pass filter before detection by the InGaAs photodiode; the system response time was 20 ls. The planar Fabry–Perot microcavities were fabricated on Si or SiO2 substrates using the thin film deposition system mentioned above. A 200-nm-thick layer of Ag was deposited via electron-beam evaporation to form a highreflectivity bottom mirror, followed by a SiO:Er layer. On one sample, a variation in thickness in the SiO:Er layer was achieved by off-axis thermal evaporation of the SiO. A thin layer of Ag was then deposited to form the output coupler. Finally, 20 nm of SiO2 were deposited as an oxidation barrier. 3. Results and discussion 3.1. Compositional analysis of SiO:Er films The microstructure of the SiO films was investigated using TEM techniques, including high-resolution electron microscopy (HREM), selected area electron diffraction,

Fig. 1. TEM analysis of an SiO film annealed for 1 h at 500 C in forming gas: (a) HREM image, (b) diffraction pattern of Si cluster, (c) EELS elemental map.

and energy filtered TEM (EFTEM). The results illustrated in Fig. 1 are for an undoped SiO film annealed at 500 C in forming gas for 1 h. The HREM image and diffraction pattern shown in Fig. 1(a) and (b), respectively, confirm that the SiO (when annealed at such low temperatures) is fully amorphous with a cluster/matrix combination. As a comparison, TEM analyses of films annealed at 1000 C or higher indicate the presence of crystalline nanoparticles [14]. The presence of amorphous Si clusters is further confirmed by Fig. 1(c), obtained by superposing the silicon (red) and oxygen (blue) EFTEM images of the SiO film.1 The mean diameter of these Si-rich clusters is 2–3 nm. The chemical composition of the Er-doped SiO films (determined by EMPA) is summarized in Table 1. The films are nearly stoichiometric SiO, but are slightly oxygen-rich due to the O provided by the Er2O3 evaporation. The abundance of oxygen in the films is important from the perspective of the 1.54 lm PL intensity, as the formation of Er–O complexes produces a defect level in the Si band wherein trapped excitons may efficiently couple their recombination energy to Er3+ ions via dipole–dipole coupling or an Auger process [12]. Such complexes also reduce the mobility and thus the segregation of Er, which can lessen concentration quenching effects [13]. 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.

A. Hryciw et al. / Optical Materials 28 (2006) 873–878 Table 1 Compositional analysis of SiO:Er films obtained using EMPA. All data are normalized to 100% Sample

Si (at.%)

O (at.%)

Er (at.%)

A B C D E

49.6 ± 0.1 49.4 ± 0.2 49.4 ± 0.3 48.3 ± 0.4 47.4 ± 0.4

50.4 ± 0.3 50.5 ± 0.3 50.3 ± 0.5 50.3 ± 0.5 50.2 ± 0.3

0.04 ± 0.02 0.09 ± 0.02 0.20 ± 0.04 1.37 ± 0.08 2.46 ± 0.08

875

The inset of Fig. 2 shows the dependence of the Er3+ PL peak intensity on annealing temperature for Sample C. The increase in intensity up to a maximum after 500 C annealing has been attributed to a reduction of defects such as dangling bonds, which could act as Er3+ non-radiative recombination centers [10,17]. Upon further increasing the annealing temperature, the intensity decreases; oxygen out-diffusion and Er precipitation or recrystallization are possible reasons for this behavior [17]. 3.3. The effect of Er concentration Fig. 3 illustrates the PL dependence on the Er concentration for SiO:Er films annealed in forming gas at

Fig. 2. Dependence of Er3+ emission on annealing process gas for a 0.20 at.% Er film annealed for 1 h at 500 C. Excitation is 40 mW of the 325 nm line of an HeCd laser. Inset: integrated Er3+ PL intensity for a 0.20 at.% Er film annealed in forming gas as a function of annealing temperature.

3.2. The effect of annealing To determine the effect of the annealing process gas on the Er3+ PL, three 0.20-at.%-Er films were annealed for 1 h at 500 C in flowing N2, Ar, and forming gas (Fig. 2). A factor of 10 increase in peak intensity for 325 nm excitation is observed after annealing in either Ar or N2, which has been attributed to a relaxation from Oh to C4v in the symmetry of the sixfold coordination of O around Er, required to optically activate the Er3+ [15]. The crystal field splitting resulting from C4v symmetry is sufficient to allow the parity-forbidden 4I13/2 ! 4I15/2 transition which produces the 1.54 lm PL. Annealing in forming gas (95% N2 + 5% H2), however, provides an increase in intensity of 20 times with respect to the as-deposited film. The ability of hydrogen to reduce alternative non-radiative recombination channels on Si nanoparticle surfaces is well documented [6]. This increase in PL intensity with hydrogenation is consistent with results for Er-doped SiO2 films with high excess silicon content [16]. Forming gas was therefore used for all subsequent anneals to maximize the Er3+ PL.

Fig. 3. Photoluminescence spectra of Samples A–E with integrated intensities vs. Er concentration shown in insets (40 mW of 325 nm excitation): (a) 1.54 lm Er3+ emission, (b) visible Si nanocluster emission. The small peak centered at 980 nm is due to the 4I11/2 ! 4I15/2 transition in Er3+, and was not included in the integrated intensity plot.

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500 C for 1 h. Three PL bands were emitted by the films: the intense Er3+-related band centered at 1535 nm (Fig. 3(a)), a much weaker band at 980 nm (due to the 4 I11/2 ! 4I15/2 Er3+ transition), and a broad band extending from 450 to 950 nm, centered at 675 nm (Fig. 3(b)). We attribute the visible/near-infrared emission to the presence of the amorphous Si nanoparticles, as confirmed by TEM; this emission is typical of silicon-rich oxide films, and consistent with the studies of films containing crystalline [5,16] or amorphous [18] nanoclusters. Upon increasing the Er concentration by a factor of 4.3 (from 0.04 to 0.20 at.%), the integrated intensity of the 1535 nm PL peak increases by a factor of 3.7; concurrently, the integrated visible peak decreases by a factor of 3. As the PL was excited at a wavelength which was not resonant with an Er3+ transition (325 nm), this behavior is consistent with the transfer of energy from excitons created in the amorphous Si nanoparticles to the Er3+ ions. Auger excitation of Er3+ to the 4I13/2 level by exciton recombination in the nanoclusters is often cited [5,19] as the excitation mechanism. In the so-called ‘‘strong coupling’’ model [20], a nanoparticle becomes ‘‘dark’’ once coupled to a neighboring Er3+ ion, given a sufficiently fast energy transfer process. Further evidence of a ‘‘strong coupling’’ mechanism is presented by the nearly complete quenching of the visible PL upon increasing the Er concentration from 0.20 to 2.46 at.%. Concurrently, the 1.54 lm PL peak decreases only by a factor of 2.5. This decrease in 1.54 lm PL for large Er concentrations may be attributed to clustering or concentration quenching effects. Further evidence of the coupling between amorphous nanoclusters and Er3+ ions is obtained from the excitation cross-sections. 3.4. PL dynamics Values for an effective excitation cross-section for the Er ions may be determined by considering the 1.54 lm PL dynamics. For a simple two-level system, the 1/e rise time (sr) and decay time (sd) are related to the excitation 1 rate by the expression R ¼ s1 r  sd [21]. 1.54 lm PL rise and decay measurements for specimens A–E are shown in Fig. 4, with rise and decay times ranging from 0.2 to 1.1 ms and 0.3 ms to 2.5 ms, respectively. The reduction in sd by approximately 30% upon increasing the Er concentration from 0.04 to 0.20 at.% is evidence of concentration quenching, which may account for the increase in the integrated 1.54 lm band intensity by only a factor of 3.7 for a concomitant increase in Er concentration of 4.3 (inset of Fig. 3(a)). From the excitation rate, we may obtain an effective Er3+ excitation cross-section which incorporates the energy transfer from the Si nanoclusters as well as non-radiative 3þ transitions, using R ¼ rEr The excitation photon eff  Uphot . 3þ 18 2 flux of 4 · 10 cm s yields rEr  1016 cm2 , similar eff to reported values for Er-doped nanocrystalline Si (e.g. Ref. [22]), and several orders of magnitude larger than 3+

Fig. 4. PL dynamics curves for Samples A–E, using 476 nm excitation. For clarity, the curves are normalized to their peak values and vertically offset: (a) PL rise curves, (b) PL decay curves.

the direct Er3+ optical absorption cross-section on the 3þ order of 1021 cm2. The calculated values for rEr eff are summarized in Table 2. Due to the non-zero system response 3þ time, these values represent a lower limit for rEr eff . These large cross-sections indicate that, as sensitizers of Er3+, the amorphous Si nanoclusters are as effective as silicon nanocrystals. Since the amorphous clusters are formed by low-temperature annealing of SiO, they offer much greater compatibility with CMOS fabrication processes. 3.5. Characterization of microcavities To demonstrate the use of SiO:Er for photonics applications, we constructed simple trial devices consisting of Fabry–Perot planar microcavities with Ag mirrors. The Er concentration in the active layer is 0.20 at.%, and the specimens were annealed at 500 C in flowing N2/H2

A. Hryciw et al. / Optical Materials 28 (2006) 873–878

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Table 2 Effective Er3+ PL dynamics and excitation cross-section results for Samples A–E

ing the linewidth. For a planar cavity, the intensity transmission coefficient is given by [23]

Sample

Decay time sd Rise time sr Excitation Excitation ±0.04 (ms) ±0.02 (ms) rate R cross-section 3þ 16 (1/s) rEr cm2 Þ eff ð10

T ð#Þ ¼

A B C D E

2.33 2.43 1.70 0.43 0.25

1.10 0.99 0.84 0.25 0.17

479 594 596 1666 1669

1.1 ± 0.3 1.4 ± 0.3 1.4 ± 0.3 3.9 ± 0.9 3.9 ± 0.9

Fig. 5. Tunable emission from Cavity A (graded SiO:Er layer thickness). Excitation was 40 mW of 325 nm. Inset: PL collected normally from Cavity B (solid line), and the resulting fit of the product of Eq. (1) and the no-cavity Er3+ PL (open symbols).

for 1 h. Cavity A has a 50-nm-thick output coupler and a graded-thickness SiO:Er layer, resulting in a smooth tuning of the resonant wavelength across the 7.5 cm length of the cavity; Cavity B has a 50-nm-thick output coupler mirror, and is tuned to the peak Er3+ emission at 1535 nm. Reflectance and transmission spectra for these structures were modelled using a matrix method for an ensemble of thin films. As illustrated in Fig. 5, the emission from Cavity A may be tuned across the entire Er3+ band (1480–1610 nm) by exciting different positions along the substrate; only 15 representative emission peaks are shown in the figure for clarity. For the cavity thickness resonant with the peak emission at 1535 nm, the full width at half maximum (FWHM) is 17 nm. For PL collected normal to the surface of Cavity B, the FWHM of the emission is 19.3 nm. The true cavity quality factor may be calculated from the expression Q = k0/Dk, where k0 is the resonant frequency and Dk is the FWHM of the mode; however, one must take into account the spectral shape of the underlying Er3+ PL band when determin-

ð1  R1 Þð1  R2 Þ ; pffiffiffiffiffiffiffiffiffiffi 2 pffiffiffiffiffiffiffiffiffiffi ð1  R1 R2 Þ þ 4 R1 R2 sin2 #

ð1Þ

where R1 and R2 are the intensity reflection coefficients of the top and bottom mirrors, respectively, and # is the optical length of the cavity. For metal mirrors a phase shift is introduced upon each reflection, yielding # = 2pdn/k + (/1 + /2)/2, where d is the physical thickness of the cavity and / are the phase shifts at the mirrors. For Cavity B, R2 may be taken as the maximum value for reflection at a SiO–Ag interface, as the bottom mirror is several times thicker than the skin depth at 1535 nm (R2 = 0.962); the value of R1, however, is subject to greater uncertainty due to the SiO2 coating of the top mirror. Fitting the product of (1) and the no-cavity PL to the cavity emission (inset Fig. 5), we obtain d = 362 nm and R1 = 0.940. The resulting transmission coefficient function has a FWHM of 26.2 nm, yielding a cavity quality factor of Q  59. Although this is lower than the quality factors of microcavities using distributed Bragg reflectors (DBRs) as mirrors, the fabrication technique using metal mirrors is much simpler; also, a wide range of tunability is possible simply by adjusting the SiO:Er layer thickness, without adjusting any mirror parameters. These strongly-emitting microcavity structures, fabricated using straightforward, low-temperature processing techniques, are made possible by the large cross-section and broadband sensitization of Er3+ in the material described. We believe these results illustrate some important practical advantages associated with the use of amorphous silicon nanoclusters as erbium sensitizers. 4. Conclusions Er-doped silicon nanocomposite thin films fabricated by co-evaporation of SiO and Er2O3 have been optimized for 1.54 lm emission with respect to Er concentration, annealing temperature, and process gas. A 0.20-at.%-Er film annealed at 500 C for 1 h exhibited the most intense Er3+ emission, compatible with standard CMOS fabrication. This makes SiO:Er a candidate for monolithically integrated optoelectronic applications. TEM analysis indicated the presence of amorphous Si nanoclusters 2–3 nm in diameter surrounded by a SiO2 matrix. From PL lifetime measurements, the effective excitation cross-section of the Er3+ was found to be 2 · 1016 cm2, similar to that of crystalline Si nanoparticles. To demonstrate the use of SiO:Er in photonics applications, we constructed planar Fabry–Perot microcavities with metal mirrors. The resonant wavelength is tunable across the entire 1.54 lm emission band by varying the SiO:Er layer thickness. A cavity resonant at the Er3+ peak of 1535 nm was found to exhibit a quality factor of 59. The control over the directionality and spectral shape of the emission afforded by such simple device structures suggests the potential of using SiO:Er as a material for integrated photonics.

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