Room-temperature photoluminescence in erbium-doped deuterated amorphous carbon prepared by low-temperature MO-PECVD

We report on a novel optical thin film material, erbium-doped deuterated amorphous carbon, fabricated directly on silicon substrate at room-temperature via controlled thermal evaporation of a Metal-Organic compound in a Plasma-Enhanced Chemical Vapour Deposition (MOPECVD) system. High erbium concentrations (up to 2.3 at.%) and roomtemperature photoluminescence at 1.54 μm are successfully demonstrated. Concentration quenching due to erbium clustering is reduced by adopting an appropriate MO precursor—Er(tmhd)3. Another quenching mechanism, caused by non-radiative C-H and O-H vibrational transitions, is shown for the first time to be significantly reduced by deuteration instead of hydrogenation of amorphous carbon. Our results suggest that erbium-doped deuterated amorphous carbon is a promising new class of photonic material for silicon-compatible optoelectronics applications in the technologically important 1.5μm wavelength region. ©2009 Optical Society of America OCIS codes: (130.3130) Integrated optics materials; (160.5690) Rare-earth-doped materials References and links 1. A. Polman, “Erbium implanted thin film photonic materials,” J. Appl. Phys. 82(1), 1–39 (1997). 2. M. E. Castagna, A. Muscara, S. Leonardi, S. Coffa, L. Caristia, C. Tringali, and S. Lorenti, “Si-based erbiumdoped light-emitting devices,” J. Lumin. 121(2), 187–192 (2006). 3. J. Lee, J. H. Shin, and N. Park, “Optical gain at 1.5 μm in nanocrystal Si-sensitized Er-doped silica waveguide using top-pumping 470 nm LEDs,” J. Lightwave Technol. 23(1), 19–25 (2005). 4. V. Toccafondo, F. Di Pasquale, S. Faralli, N. Daldosso, L. Pavesi, and H. E. 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Introduction
Integration of electronic and optical communication devices on a single silicon chip is challenged by silicon's inability to readily emit or amplify light on the one hand, and its incompatibility with predominately III-V high speed photonic devices on the other.Realization of electrical and optical functions co-existing on a single chip requires the development of silicon-compatible light emitting materials.Due to Er 3+ emission in the 1.5 µm region, the wavelength preferred by the majority of optical communication devices, erbium doping in a variety of silicon-based materials has been investigated [1] for their potential application in integrated optoelectronics.Most recent breakthroughs include the demonstration of electroluminescence in erbium-doped silicon-rich silicon oxide (SRSO(Er)) light-emitting diodes [2], the observation of optically pumped gain and amplifier characteristic in SRSO(Er) waveguides [3,4], and the realization of an ultra-low threshold in optically pumped micro-laser based on erbium-implanted SiO 2 [5].Extensive efforts in seeking promising host candidates are still going strong.Other than crystalline silicon and silica, it was shown that photo-or electro-luminescence at 1.54µm also exists in hydrogenated amorphous silicon [2], in silicon carbide [6,7], and in Er silicate [8], and more recently, optical gain was observed in Yb 3+ -sensitized Er-doped porous silicon [9].
Notwithstanding the extensive research on Er-doped silicon-based materials, Er-doping in carbon materials has received little scrutiny, despite the fact that carbon-based materials are compatible with silicon substrates and offer a number of advantages as a host.Take hydrogenated amorphous carbon (a-C:H) as an example, which can be readily prepared using a low-temperature plasma enhanced chemical vapour deposition (PECVD) process.(Here, hydrogenation of amorphous carbon stabilizes the structure by terminating the π dangling bonds.)This material is promising because of its high film quality, its easy integration with current metal-oxide-semiconductor (CMOS) technology, and its low cost and reproducibility.Moreover, it is feasible to obtain a-C:H films with a wide range of opto-electronic properties by adjusting the deposition parameters in the growth process.In particular, a-C:H offers the following desirable properties [10]: (i) it can be directly deposited on silicon substrate and easily etched by an oxygen plasma, allowing easy integration; (ii) it has large tailorable optical band gap (0.9 -4.3 eV) covering from IR to visible; (iii) its conductivity can be altered by two orders of magnitude (5×10 3 -5×10 5 S m −1 ) through p-or n-doping; (iv) its refractive index can be altered from 1.47 -2.76, providing more flexibility in waveguide design and mode confinement.These flexible opto-electronic properties of a-C:H allows one to tailor the host material for specific optoelectronic applications.
Major difficulties with obtaining light emission near 1.54 µm in Er-doped a-C:H films is due to the severe quenching of radiative emission by the C-H and O-H vibrational modes [11,12].Herein we report, for the first time, that deuteration, instead of hydrogenation, can effectively overcome the quenching of ErP 3+ P luminescence caused C-H and O-H bonds.Erbium-doped deuterated amorphous carbon (a-C:D(Er)) films are fabricated directly on silicon substrate at room-temperature via an MO-PECVD system.We show that high erbium concentrations and significantly enhanced room-temperature photoluminescence (PL) at 1.54 µm can be obtained in a-C:D(Er).Our results suggest that a-C:D(Er) is a promising photonic material for silicon-compatible optoelectronics applications.

The metal-organic precursor
One important concern regarding the host material for erbium is its ability to reduce concentration quenching effects associated with erbium ion clustering at high concentrations.Such quenching effects limit the optical gain that can be obtained from Er-doped hosts, as a result of co-operative upconversion and energy migration [13,14].At high erbium concentrations, an Er ion in the excited state is more likely to release its energy nonradiatively to cause an upward transition in a nearby Er ion, a process known as co-operative upconversion, than to de-excite radiatively.The upconversion process is then likely to repeat through a chain of ion-ion interactions, resulting in energy migration in a host until a quenching centre is encountered, where the energy is dissipated non-radiatively.This quenching process causes a decrease in luminescence efficiency [15].
To reduce concentration quenching, we selected tris(2,2,6,6-tetramethy1-3-5heptanedionato) Erbium(III), abbreviated Er(tmhd) 3 , as the metal-organic precursor in our MO-PECVD process.Its chemical composition is Er(C 11 H 19 O 2 ) 3, which is illustrated in Fig. 1.Under appropriate deposition conditions, the Er(tmhd) 3 molecule can be incorporated into the host material while preserving the Er-O bonds, as well as the long carbon chains (ligands).Thus, large separation between erbium ions is ensured, and accordingly, Er-Er co-operative upconversion is reduced.Moreover, the hydro-carbon ligands provide the framework for seamless integration into a hydrogenated (or deuterated) amorphous carbon network, allowing high solubility of erbium.Erbium concentration as high as 8.74 at.% in carbon films was reported [16].Furthermore, the ligands in Er(tmhd) 3 act as sensitizers, which absorb optical excitation energy and transfer it to the encapsulated Er ion [17][18][19][20].Thus, the effective absorption cross-section of Er 3+ is increased, making it more amenable to optical pumping.
Other advantages of using Er(tmhd) 3 as a precursor include: (i) Er(tmhd) 3 has a high vapour pressure of 0.1 mm Hg at 160 °C [21], allowing us to deliver a controlled evaporant using a low-temperature deposition technique; (ii) unlike ion-implantation, which is expensive and creates film damage during the implantation process, our low-temperature MO-PECVD technique does not require subsequent high-temperature annealing to repair film damage; (iii) each Er(tmhd) 3 molecule contains an erbium ion in the Er 3+ form (which emits at 1.54mm) surrounded by six oxygen atoms [22], which can be directly incorporated in the host material without needing a post-growth process to activate erbium; and, (iv) Er(tmhd) 3 does not give rise to contamination problems often associated with using other metal-organic sources for Er [23].

Hydrogenation versus deuteration
Another important issue concerning a host for Er is the existence of non-radiative deactivation channels intrinsically within the host material.For amorphous carbon, it is well known that C-H vibrations in the vicinity of Er 3+ play an important role in quenching the luminescence lifetime of Er 3+ [11], and hydroxyl groups (O-H) was also shown to resonate with the Er 3+ 1.5 µm emission in silicate glass [12].A systematic comparison between Erimplanted silicate glasses with different O-H impurity contents showed a correlation between O-H content and luminescence lifetime [24].In Fig. 2, it can be clearly seen that the radiative transition in Er 3+ (~6500 cm −1 ), between the ground state 4 I 15/2 and the first excited state 4 I 13/2 , approximately matches the second harmonics of C-H and O-H bond vibrations (5900 and 6900 cm −1 , respectively) of the host material.Hence, excited Er ions can efficiently perturb the nearby C-H or O-H oscillators, resulting in a non-radiative transition.It is therefore not surprising that so far only one publication, by Speranza et al. [25], reported a study of Er luminescence in a a-C:H host, observing very weak Er photoluminescence (PL) despite applying a high optical excitation power on a sample of high Er concentration (~1.2 at.%).
To effectively suppress this inherent quenching of Er emission by C-H and O-H vibration modes, in this study, we remove the C-H bonds in the host by substituting hydrogen atoms with heavier deuterium atoms (H → D), consequently presenting a first demonstration of significantly enhanced room-temperature photoluminescence in erbium-doped deuterated amorphous carbon (a-C:D(Er)) films.As can be seen from Fig. 2, deuteration modifies the harmonic number, ν, of the transitions that overlap with the radiative transition of Er, from ν = 2 (for C-H and O-H) to ν = 3 (for C-D and O-D).The interaction strength between Er 3+ and the third harmonic of C-D vibrations, is much weaker than that between Er 3+ and the second harmonic of C-H vibrations [26].Therefore transition probability between Er 3+ and the vibration modes of the host material is reduced through deuteration.The transition probabilities of ν = 2 and ν = 3 can be quantitatively compared by adopting the Franck-Condon factor, F, with an approximation of the undistorted oscillator model [27]: where k is force constant, q and 0 q are equilibrium positions of the oscillators, £ is Planck's constant, and ω is frequency.Equation (1) suggests that the factor F decrease as ν increases.
To verify experimentally the effectiveness of suppressing the quenching effect by replacing H with D, simple PL measurements were carried out to compare the Er emission from 1.1 mol% of Er(tmhd) 3 diluted in methanol and that from the same concentration of Er(tmhd) 3 diluted in deuterated methanol.With 86% of the hydrogen replaced by deuterium, the peak PL intensity is enhanced by ten-fold, as seen in Fig. 3(a).Moreover, we compare the PL intensity of an erbium-doped hydrogenated amorphous carbon (a-C:H(Er)) film, prepared using methane (CH 4 ) as a precursor gas, and that of an erbium-doped deuterated amorphous carbon (a-C:D(Er)) film, prepared under the same deposition conditions except the precursor gas was replaced by deuterated methane (CD 4 ).(Details on sample preparation and characterization are provided in the next two sections.)As in the cases for the Er(tmhd) 3 solutions, a ten-fold enhancement in Er PL is observed (Fig. 3     As seen in Fig. 6 (a), room-temperature PL spectra peaked at 1540 nm, corresponding to the 4 I 13/2 → 4 I 15/2 transition of Er 3+ and its full width at half-maximum (FWHM) is about 70 nm.This FWHM is wider than those of other Er-doped silicate glasses, suggesting that Er 3+ have different local environments in the amorphous carbon matrix.The wider emission bandwidth indicates the potential of enabling a wide gain bandwidth for optical amplification.In order to verify that the majority of the incorporated erbium is in the optically active form of Er 3+ , surrounded by oxygen atoms (as illustrated in Fig. 1), binding energy (BE) analysis using XPS was carried out.The shape of the BE spectrum is indicative of the oxidation state of Er in the material, and the BE spectra of the Er(tmhd) 3 powder, the evaporated film of Er(tmhd) 3 , and all a-C:D(Er) samples were compared.The normalized 4d spectra of Er are given in Fig. 7, showing a characteristic spectral feature near 169 eV.Such feature is attributed to the 4d levels in Er 3+ forming a multiplet through an interaction with the unfilled shell [32].Spectral multiplets at ~169 eV with essentially the same features as shown in Fig. 6 were observed in the 4d photoemission spectra obtained from the erbium state of 4f 11 /Er 3+ [32], as well as from Er 2 O 3 [33].The existence of the multiplet near 169 eV for all the cases we measured suggests that the oxidation state of Er in Er(tmhd) 3 powder did not change markedly during evaporation and the deposition of the Er(tmhd) 3 film, nor did it change significantly during the MO-PECVD process.We therefore conclude that the incorporation of Er into the a-C:D(Er) samples essentially preserves the Er 3+ state, forming efficient emission centers.Fig. 7. Er 4d XPS spectra of the four a-C:D(Er) film samples, an Er(tmhd)R3R film (prepared by evaporating the powder) and the Er(tmhd)R3R powder.Each spectrum is normalized to its maximum intensity after a background subtraction and offset vertically for clarity of presentation.

Conclusion
We have demonstrated significantly enhanced room-temperature Er photoluminescence in a-C:D(Er) thin films deposited by metal-organic PECVD.Our simple fabrication technique offers four essential advantages of the a-C:D(Er) material: (i) controllable and uniform Er concentration as large as 2.3 at.%, the highest reported in amorphous carbon as a host material; (ii) the possibility of obtaining a wide range of tailorable optoelectronic properties; (iii) the elimination of the annealing step (a required step in many Er-doped materials); and, (iv) deuteration for suppressing quenching.All these concur in an easy one-step film growth procedure.Film thicknesses up to 2000 nm have been achieved using this technique.No concentration quenching effects have been observed up to 1.4 at.%Er 3+ .Binding energy analysis confirmed that the Er ions in a-C:D(Er) having the optically active oxidation state of Er +3 can be incorporated at room-temperature.It has also been shown that deuteration of amorphous carbon has effectively removed the non-radiative second order C-H and O-H vibrational modes, resulting in a significant enhancement in PL at 1.5 µm in a-C:D(Er) in contrast to that in a-C:H(Er).The efficient Er emission in a-C:D(Er) films, along with its wide range of tailorable conductivity, optical bandgap and refractive index, suggests a-C:D(Er) is a promising material for realizing integrated light-emitting and light-amplifying devices using CMOS technology.

Fig. 1 .
Fig. 1.Illustration of the erbium metal organic precursor, tris(2,2,6,6-tetramethy1-3-5heptanedionato) Erbium(III), abbreviated Er(tmhd)3.The large central atom (purple) represents Er, the immediately surrounding 6 atoms (red) represent O, the larger atoms (dark grey) attached to the oxygen atoms are C atoms, while the smaller atoms (light grey) attached to carbon atoms represent H atoms. Note, the hydro-carbon ligands provide the framework for seamless integration into a hydrogenated/deuterated amorphous carbon network.

Fig. 2 .
Fig. 2. Illustration of energy levels of vibrational modes in organic media and the broadened 4 I13/2 → 4 I15/2 transition of Er 3+ ; ν represents the harmonic numbers.The data concerning the C-H, C-D, O-H, and O-D vibrational modes are taken from [26].The arrows indicate the energy transfer from excited Er 3+ to the matching vibrational modes.The different styles of the arrows (bold compared to dashed) indicate the transition probability, which is higher for C-H and O-H at ν = 2 (bold) than for C-D and O-D at ν = 3 (dashed).
(b)), suggesting that most of the #116103 -$15.00USD Received 24 Aug 2009; revised 16 Oct 2009; accepted 19 Oct 2009; published 4 Nov 2009 (C) 2009 OSA C-H bonds are similarly replaced by C-D bonds in the deuterated film.This result confirms that Er luminescence efficiency can be dramatically improved through deuteration of amorphous carbon in a one-step deposition procedure.

Fig. 3 .
Fig. 3. (a) PL comparison between 1.1 mol% of Er(tmhd)3 diluted in methanol (CH3OH) and deuterated methanol (CD3OD).The peak intensity is improved by ten-fold when Er(tmhd)3 is dissolved in C-H free solvent.(b) PL comparison between a-C:H(Er: 2.0%) and a-C:D(Er: 2.3%) films prepared under same deposition conditions except for the different precursor gas, CH4 for a-C:H(Er) and CD4 for a-C:D(Er).By deuteration of host material, the intensity of Er PL at 1540 nm is enhanced by ten-fold.

Fig. 4 .
Fig. 4. A schematic diagram of metal organic -dc saddle-field plasma enhanced chemical vapour deposition system used for the preparation of erbium doped deuterated amorphous carbon.The grey region surrounding the semi-transparent electrodes (mesh) represents the deuterated methane plasma.

Fig. 5 .
Fig. 5. C, Si, O, Er and O concentrations as function of the distance from the film surface as determined by XPS for sample #4.

Fig. 6 .
Fig. 6.(a) Room-temperature PL spectra of a-C:D(Er) samples with different Er concentrations and film thicknesses.The peak is centered at 1540 nm with FWHM of ~70 nm.(b) The peak PL intensity normalized to film thickness as a function of Er concentration in a-C:D(Er) films.The symbols are actual data points, the line is a guide to the eye.The linear region suggests no concentration quenching for Er concentrations up to at least 1.4 at.%.To study the concentration dependence of the Er luminescence efficiency in more detail, the peak PL intensity is normalized to film thickness, denoted by I nor , and then plotted as a function of Er concentration N Er .Under cw laser excitation, I nor is proportional to σφNτ/τ rad , where σ is the excitation cross section, φ is the photon flux, N is the optically active Er