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Atomic layer engineering of Er-ion distribution in highly doped Er:AlO for photoluminescence enhancement 2

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John Rönn, Lasse Karvonen, Christoffer Joonas Kauppinen, Alexander Pyymaki Perros, Nasser Peyghambarian, Harri Lipsanen, Antti Saynatjoki, and Zhipei Sun ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00283 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Atomic layer engineering of Er-ion distribution in highly doped Er:Al2O3 for photoluminescence enhancement John Rönn,∗,† Lasse Karvonen,† Christoffer Kauppinen,† Alexander Pyymaki Perros,† Nasser Peyghambarian,‡,†,¶ Harri Lipsanen,† Antti Säynätjoki,†,¶ and Zhipei Sun† †Department of Micro- and Nanosciences, Aalto University, Espoo, Finland ‡College of Optical Sciences, University of Arizona, Tucson, USA ¶Institute of Photonics, University of Eastern Finland, Joensuu, Finland E-mail: [email protected];[email protected] Phone: +358 (0)503549262

Abstract

hance the photoluminescence of our Er:Al2 O3 material by up 16 times stronger when compared to the case where the Er-concentration is ∼ 0.6%. In addition, long lifetime of approximately 5 ms is preserved in the Er-ions even at such high concentration levels. Thus, our optimized ALD process shows very promising potential for the deposition of optical gain media for integrated photonics structures.

For the past decade, erbium-doped integrated waveguide amplifiers and lasers have shown excellent potential for on-chip amplification and generation of light at the important telecommunication wavelength regime. However, Erbased integrated devices can only provide small gain per unit length due to the severe energytransfer between the Er-ions at high concentration levels. Therefore, active ion concentrations have been limited to < 1% levels in these devices for optimal performance. Here, we show an efficient and practical way of fabricating Erdoped Al2 O3 with Er-concentration as high as ∼ 3.5% before concentration quenching starts to limit the C-band emission in our material. The Er-doped Al2 O3 was fabricated by engineering the distribution of the Er-ions in Al2 O3 with the atomic layer deposition (ALD) technique. By choosing a proper precursor for the fabrication of Er2 O3 , the steric hindrance effect was utilized to increase the distance between the Er-ions in the lateral direction. In the vertical direction, the distance was controlled by introducing subsequent Al2 O3 -layers between Er2 O3 -layers. This atomic scale control of the Er-ion distribution allows us to en-

Keywords Erbium, rare-earth ions, atomic layer deposition, photoluminescence, optical amplifier, integrated photonics.

Recently, rare-earth-ion-doped (REID) materials have drawn a significant attention in integrated photonics as an alternative solution for on-chip amplification and generation of light over III-V semiconductor compounds. 1,2 The unambiguous advantages of using REID materials in integrated photonics are vast. For example, REID materials can be integrated in silicon photonics technology with no major chal-

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lenges; 3–5 their long excited state lifetime enables them to be operated in high bit-rate systems; 6 and their fabrication is straightforward and cheap. 7 Of the available rare-earth ions, erbium has been studied the most by virtue of its broad emission band at the important C-band wavelength regime, which has led to the success of erbium-doped fiber amplifiers and lasers. 8 However, in order to use erbium as a gain material in integrated photonics efficiently, one has to apply high concentration of Er-ions (>1020 cm−3 , ∼ 0.1%) in order to provide sufficient amplification over short integrated waveguide structures. The realization of highly doped gain medium with acceptable efficiency is extremely challenging. For example, silica, the most common and reliable optical host material, cannot be used as host for the Er-ions due to its low solubility. 9 In addition, the high incorporation of ions leads to concentration quenching and creation of unwanted transitions that ultimately reduce the gain and increase the noise in the Er-doped amplifier system. 10 Therefore, new materials and novel fabrication techniques are urgently needed to answer these challenges and to optimize these newgeneration active devices for integrated photonics, e.g., silicon photonics industry. In literature, plethora of fabrication methods have been successfully employed to deposit Er-doped materials, including atomic layer deposition, 11 various chemical vapor deposition methods, 12–14 flame hydrolysis, 15 pulsed laser deposition, 16 reactive co-sputtering, 6 RFsputtering, 17 sol-gel method, 18 spin coating 19 and dip-coating. 20 In addition, ex-situ methods, such as ion implantation 21 and in-diffusion 22 have also been demonstrated. For the choice of proper host material, amorphous Al2 O3 has been shown to be excellent for high concentration of Er-ions due to its similarity in valency and lattice constant with Er2 O3 . 23–25 Al2 O3 also possesses relatively high refractive index (n ∼ 1.65) as compared to for example SiO2 (n ∼ 1.45), which allows tighter light confinement in Er:Al2 O3 waveguide structures. For these reasons, Al2 O3 has found its way as one of the most common and reliable host materials for Er-based integrated waveguide devices. 4–6

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Thus far, the focus of depositing optically active Er-doped Al2 O3 has mainly been on reactive co-sputtering, pulsed laser deposition and ion implantation techniques. 21,24,26 Although all of these methods can be used to incorporate high concentrations of Er-ions into Al2 O3 , none of these techniques can be directly used to control the sub-nanometer scale spatial distribution of the Er-ions within the Al2 O3 host. The control of the Er-ion profile is important because not only the high concentration, but also the local distribution of active ions has been shown to affect the energy-transfer probability in the Er-doped material. 27,28 In contrast to commonly used fabrication methods, ALD is an advanced deposition technique that deposits thin films atomic layer at a time, which enables a variety of advantages such as very high conformity, accurate control of the film thickness and ability to deposit multilayer films. 29 These advantages are very unique when considering a suitable fabrication method for Er-doped integrated waveguide devices, as at least the following properties can be fulfilled: (i) ALD can be used to fabricate the host material while performing the doping of the Er-ions at the same time and in a single process run; (ii) the doping of the Er-ions can be done very delicately as the atomic scale spatial distribution of the Er-ions can be controlled and (iii) the gain material can be deposited at relatively low temperature on a ready-made device without needing to perform additional lithography steps. In the present work, we show that ALD is an exceptional technique to fabricate highly doped (≥ 1%) Er:Al2 O3 gain media by using its subnano scale engineering ability to carefully control the incorporation of Er-ions into Al2 O3 during the deposition. We demonstrate this by fabricating Er:Al2 O3 thin films with different spatial distributions of Er-ions within the Al2 O3 host. With detailed photoluminescence (PL) characterization of the fabricated samples, we verify that our technique is capable of incorporating up to 3.5 % of Er-ions into Al2 O3 before severe concentration quenching occurs. The optimization of the Er-ion profile enables us to enhance the peak photoluminescence response of our Er:Al2 O3 by up to 1580% when com-


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ACS Photonics

to be at least 260-325 ◦ C, which also overlaps the ALD window for Al2 O3 . 32 However, we decided to deposit the Er-doped Al2 O3 samples at relatively high temperature (300◦ C) to minimize the amount of impurities in the film but still avoiding the decomposition of the TMA precursor. At 300◦ C, the average growth rates were found to be 0.20 and 0.85 Å/cycle for Er2 O3 and Al2 O3 , respectively. In order to optimize the performance of the Er-doped Al2 O3 material as an active medium, we set the following goals for it: (i) to contain as high active Er-concentration as possible; (ii) manage the co-operative up-conversion that results from the energy-transfer of the Er-ions at high concentration levels; and (iii) preserve the long lifetime of the first excited state in the Erions. For this purpose, we prepared a total of 20 Er:Al2 O3 samples with four different spatial distribution of Er-ions by keeping the amount of Er2 O3 -layers constant but varying the number of Al2 O3 -layers in each of these four processes. The fabrication procedure went as follows: at first, we prepared one Er:Al2 O3 sample with 1:10 Er2 O3 :Al2 O3 sequence, which corresponds to approximately 0.6 % atomic concentration of Er in Al2 O3 . The average interionic vertical distance (d|| ) in this structure was deduced to be 8.3 Å, which was estimated from the thickness of the Al2 O3 -layer in each ALD super cycle. In the second sample, d|| was halved by reducing the number of Al2 O3 -layers to 5. Finally, two more samples were produced in which the Er-concentration was further increased by first reducing the number of Al2 O3 layers to 3 and then to 2. The lateral interionic distance (d= ) within one Er2 O3 -plane in the fabricated samples was roughly estimated from the growth rate of Er2 O3 and the structure of the Er(thd)3 molecule. 33 The fabrication process of the samples is demonstrated in Fig. 1c and Table 1 gives the fabrication related properties of the deposited samples. The elemental composition of the as-deposited samples was determined with energy-dispersive Xray spectroscopy (FEI Helios Nanolab600 with an integrated EDX detector manufactured by EDAX) by focusing high energy (15 keV) beam of electrons into the samples. The EDX system

pared to the case where the Er-concentration is ∼ 0.6%. In addition, we demonstrate that long excited state lifetime of approximately 5 ms is maintained in the Er-ions even at such high concentration levels.

Experimental methods Sample Preparation. Er-doped Al2 O3 was produced with Picosun R-200 ALD tool operating in the plasma-enhanced mode by sequentially depositing Er2 O3 and Al2 O3 to form a nanolaminate stack on top of a silicon substrate. Tris(2,2,6,6-tetramethyl3,5-heptanedionato)erbium (also known as Er(thd)3 or Er(tmhd)3 ) and oxygen plasma were chosen for the deposition of Er2 O3 , whereas trimethylaluminium (TMA) and water were chosen for Al2 O3 . The Er(thd)3 precursor is an organometallic compound with the chemical formula of Er(C11 H19 O2 )3 , as illustrated in Fig. 1a. Although there are six oxygen atoms in the precursor molecule, erbium is still found in its trivalent, Er3+ state, since the erbium-ligand connection is made via partial bonds between the Er-ion and two of the oxygen atoms in the ligand. Due to relatively large size of the precursor molecule and the nature of its bulky ligands, the steric hindrance effect forces relatively large lateral interionic distance between the Er-ions in the Er2 O3 -layer during its fabrication. 30 This is very advantageous in the context of gain material design, as it decreases the energy-transfer probability between the adjacent Er-ions. 31 Fig. 1b demonstrates how one cycle of Er2 O3 is deposited and how the steric hindrance affects the growth of the film during the deposition. In addition, the vertical interionic distance between the Er-ions (i.e. the distance between subsequent Er2 O3 layers) was controlled by adjusting the Al2 O3 interlayer thickness between each Er2 O3 -layer. Prior to the deposition of Er:Al2 O3 samples, the ALD process for Er2 O3 was optimized by maximizing the growth rate and minimizing the non-uniformity of the Er2 O3 film. The ALD window (i.e. the temperature range for constant deposition rate) for this process was found


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a)

c)

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x cycles

Al 2O3

1 cycle

Er2O3

500 cycles

b)

Step 2: Purge

Step 1: Er(thd) exposure Steric hindrance increases interionic distance between Er-ions

Chemisorption: At least one of the ligands in Er(thd) is removed and bond between Er and O is formed.

By-products are removed via purge. Steric hindrance effect prevents close-packing of Er -ions.

Step 4: Purge

Step 3: O-plasma exposure

The plasma either removes or burns the remaining ligands and creates new surface sites for subsequent reactions.

Purge of the remaining by-products. One submonolayer of erbium oxide is formed.

Figure 1: a) Composition of the Er(thd)3 molecule. The central ion corresponds to erbium, whereas red, grey and white correspond to oxygen, carbon and hydrogen atoms, respectively; b) Schematic presenting the formation of single Er2 O3 layer with ALD; c) Fabrication procedure for each sample. The samples consist of one ALD cycle of Er2 O3 and x ALD cycles of Al2 O3 , repeated by 500 times. Increasing x increases vertical interionic distance between the Er-ions. The value of x varies from 2 to 10.


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Table 1: Sample properties and their preparation conditions. Number of cycles

Elemental composition (at. %)

Sample

Er2 O3

Al2 O3

Total

Thickness (nm)

O

Al

Er

d|| (Å)

1 2 3 4

1 1 1 1

10 5 3 2

500 500 500 500

494.4 242.3 131.6 89.9

53.6 54.8 53.1 51.9

45.8 43.7 43.4 44.0

0.6 1.5 3.5 4.1

8.3 4.2 2.5 1.7

d= (Å) > > > >

4.4 4.4 4.4 4.4

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I15/2 ) of all the fabricated samples were studied and compared using the fiber-coupled 980 nm laser operating at constant excitation intensity of ∼ 0.5 · 104 W/cm2 . The purpose of this step was to experiment how the annealing temperature affects the PL response of the samples. In the second step, the UC spectra of the sample set with the strongest PL yield (based on the post-process annealing conditions) were measured using the same 980 nm laser source. Finally, the optimal sample set was further characterized with pump-power and time-decay measurements using the frequencydoubled Nd:YAG laser in order to determine their performance as an optical amplifier. In the time-decay measurements, the frequency of the chopper was changed to 40 Hz and the PL signal was collected during the dark time of the excitation.

was calibrated by measuring plain Al2 O3 and Er2 O3 films and confirming them to be stoichiometric. In addition, four replica of each fabricated sample were produced, which were post-process annealed for 30 min in nitrogen atmosphere at 600, 650, 700 and 750 ◦ C in order to investigate the effect of the annealing temperature on the PL response of the samples. This specific temperature range has been reported to enhance the PL response of Er-doped Al2 O3 either by increasing its excited state lifetime or the number of active ions present in the material without redistributing the Er-ions in Al2 O3 . 34 Photoluminescence characterization. We characterized the photoluminescence response of the fabricated samples by performing both time-independent and time-dependent room-temperature PL measurements at infrared region (λ = 1460 − 1580 nm). In addition, PL measurements were also performed at visible wavelengths (λ = 500 − 800 nm) in order to get qualitative information whether co-operative up-conversion (UC) occurs in the samples. Two distinct laser sources were used in the study: a fiber-coupled diode laser operating at 980 nm and a frequency-doubled Nd:YAG laser operating at 532 nm. In the optical setup, the laser light was transmitted through a chopper operating at 190 Hz and then directed onto the sample. The emitted light from the sample was collected and then sent into a monochromator that was connected into a InGaAs detector. The photocurrent of the detector was measured as a function of the wavelength with a lock-in amplifier in order to increase the sensitivity of the system. The PL characterization was done in three steps. At first, the PL responses (4 I13/2 →

Results and discussion Effect of Annealing. Figs. 2a-c present the room-temperature PL spectra of the asdeposited (300 ◦ C) samples, the samples annealed at 600 ◦ C and the samples annealed at 750 ◦ C, respectively. Firstly, all the samples exhibit broad emission spectra peaked at λ = 1533 nm, which can be related to the characteristic (I13/2 → I15/2 ) transition in Er-ions. 8 In the case of the as-deposited samples, the PL signal shows an increasing trend when the Er-concentration is increased from 0.6% (Sample 1) to 4.1% (Sample 4) without any signs of PL quenching. This PL enhancement can be directly related to the increase in the optically active Er-ions when the Er-concentration is increased. When the samples are annealed


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at 600 ◦ C, the annealing is, in fact, detrimental for Samples 1 and 2 and only slightly beneficial for Samples 3 and 4. Similar decrease in the PL yield has been observed before when Er:Al2 O3 samples were annealed at 300-500 ◦ C, which was accounted as a consequence of hydrogen desorption in the films. 35 Nevertheless, when the samples are annealed at 750 ◦ C, the PL yield of all the samples can be greatly enhanced. In order to study and compare how the annealing temperature affects the PL response of the samples with different Er-concentrations, the PL spectra (I13/2 → I15/2 transition) were integrated. Fig. 2d presents the integrated PL response of all the annealed samples, including the samples annealed at 650 ◦ C and 700 ◦ C, as a function of the Er-concentration. Interestingly, Fig. 2d reveals that the strongest PL yield is obtained from Sample 3 (3.5 % Er), rather than from Sample 4 (4.1 % Er), when the annealing temperature is highest.. Thus, we can make two considerable remarks from these measurements. Firstly, the post-process annealing, although not required to optically activate the Er:Al2 O3 samples, can greatly improve the PL performance of the samples if the annealing temperature is chosen properly. Secondly, the PL spectra of the annealed samples show saturating behavior in the case of Sample 4, indicating the presence of PL quenching. Since the PL quenching does not occur in the case of the as-deposited samples, they must contain lower number of active Er-ions. Therefore, we can conclude that the annealing at > 600 ◦ C has a beneficial effect of increasing either the number of optically active Er-ions and/or the excited state lifetime of the samples, which is the reason for the enhancement of the PL intensity between the as-deposited samples and the samples annealed at 750 ◦ C. Up-conversion. In order to investigate the origin of the PL quenching that occurs in the samples with the highest Er-concentration, we studied the up-conversion spectra of the 750 ◦ C sample set at visible wavelengths (λ = 500 − 800 nm) using the same fiber-coupled 980 nm laser source. In the studied wavelength regime, only the characteristic green (4 S3/2 → 4 I15/2 )

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up-converted light emission was observed at around 543 nm, which has also been reported before in highly-doped Er:Al2 O3 . 10 With 980 nm excitation, there are two mechanisms in the Er-ions that can produce the observed UC signal: excited state absorption (4 I15/2 → 4 I11/2 → 4 F7/2 ) and co-operative up-conversion (4 I9/2 + 4 I9/2 → 4 I15/2 + 4 F7/2 ). The former of these mechanisms is proportional to the excitation intensity and the population of Er-ions in the 4 I11/2 state, whereas the latter is proportional to the square of the population of Er-ions in the 4 I11/2 state and the UC coefficient of the specific UC process. 9 The measured UC spectra and the integrated UC yield of the samples are presented in Figs. 3a and b, respectively. Figs. 3a and b show that the increase in UC intensity is relatively small when the Er-concentration is increased from 0.6 % to 3.5 %. However, rapid increase in the UC intensity occurs when the Er-concentration is increased beyond the 3.5 % level. Thus, it appears that strong energy-transfer occurs in Sample 4. Although we cannot readily confirm whether the origin of the measured UC signal is excited state absorption or co-operative up-conversion, the nonlinear nature of the UC yield strongly suggests that the UC is either from co-operative up-conversion alone, or from the combination of co-operative up-conversion and excited state absorption. Thus, the cooperative up-conversion is most likely the reason for the PL quenching in Sample 4. Unfortunately, we could not measure the exact probability (C37 ) for the co-operative up-conversion process since our excitation intensity was not strong enough to do a thorough analysis of the samples. Instead, we proceeded to do comprehensive pump-power and time-decay characterization of the 750 ◦ C sample set with 532 nm excitation light source in order to avoid the excited state absorption and to get access to higher excitation intensities. Time-decay and pump-power analyses. The time-decay and pump-power analyses allow us to study the amplification properties of the optimal sample set by determining their lifetime and probability for the co-operative upconversion process (4 I13/2 + 4 I13/2 → 4 I15/2 +


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a)

1.5 % Er

3.5 % Er 3

300 °C PL yield (arb. unit)

PL yield (arb. unit)

2.5 2 1.5 1 0.5

0 1480

1500

1520

1540

1560

4.1 % Er

b)

600 °C

2.5 2 1.5 1 0.5 0 1480

1580

1500

Wavelength (nm) 18

c)

1540

1560

1580

d)

750 °C

15 12 9 6 3 0 1480

1520

Wavelength (nm)

Integrated PL yield (arb. unit)

PL yield (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12

750 °C 700 °C

9

650 °C

6 3

600 °C 0 1500

1520

1540

1560

1580

0

Wavelength (nm)

1

2

3

4

Er-concentration (at. %)

Figure 2: PL spectra of a) the as-deposited samples; b) the samples annealed at 600 ◦ C and c) the samples annealed at 750 ◦ C at the wavelength range λ = 1460 − 1580 nm; d) Integrated (I13/2 → I15/2 ) PL intensity of all the annealed (16) samples as a function of the Er-concentration. The spectra have been divided with the corresponding sample thickness and then normalized to the PL response of Sample 1 (as-deposited) for comparison. 4

I9/2 ) that occurs in the first excited state of the Er-ions. In fact, it is this co-operative upconversion process that greatly limits the maximum gain in the Er-doped waveguide amplifiers at high Er-concentrations. In order to measure the strength of this specific up-conversion process in our samples, the lifetime of the first excited state in the Er-ions needs to be known. The lifetimes of the 750 ◦ C sample set were measured with conventional time-decay analysis by pumping the samples to steady-state with 532 nm excitation (I = 0.62 · 104 W/cm2 ), turning off the pump source and recording the PL intensity at 1.53 µm as a function of time. The lifetimes of the as-deposited samples were also measured for comparison. Fig. 4a. demon-

strates the case of Sample 3, with triangles presenting the PL intensity of the as-deposited sample and circles presenting the PL intensity of the annealed sample as a function of time. A typical exponential decay of the PL intensity can be observed for both samples. The decay characteristics of the PL intensity can be modeled theoretically by solving the rateequations that govern each sample with specific pump intensity. The details of this theoretical analysis are presented in the Supporting information section. By fitting Eq. S6 to the experimental data, with τ2a and C24 N2a (0) as the fitting parameters, we find τ2a = 2.10 ms for the as-deposited sample and τ2a = 5.04 ms (C24 N2a (0) = 70.19) for the annealed sample.


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a)

b)

15 12

18 15

UC yield (arb.unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Integrated UC yield (arb.unit)

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0.6 % Er 1.5 % Er 3.5 % Er 4.1 % Er

9 6 3 0 520

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9 6 3 0

530

540

550

560

1

Wavelength (nm)

2

3

4


Figure 3: a) Up-conversion spectra of the samples annealed at 750 ◦ C at the wavelength range λ = 520 − 565 nm and b) Integrated UC yield as a function of the Er-concentration. The UC signal has been divided with the corresponding sample thickness and then normalized to the maximum UC signal of the 750 ◦ C sample set for comparison. The same analysis was performed for the remaining samples in the 750 ◦ C sample set and also for the as-deposited samples. We found out that the lifetimes did not deviate dramatically between the samples in the same sample set (i.e. as-deposited or annealed), with only slight decline as a function of the Er-concentration in the samples. In addition, we observed that a constant lifetime increase of approximately 3 ms could be obtained when the as-deposited samples were annealed at 750 ◦ C. Constant increase in the lifetime indicates the removal of defects (e.g. voids) and impurities, such OH- and CHgroups, that give rise to non-radiative quenching sites for the Er-ions. Although we were unable to determine the presence or the exact content of the carbon and hydrogen impurities in the samples, we believe that they constitute only a small fraction of the film composition, since the ligand removal in both Er2 O3 and Al2 O3 was ensured during their individual ALD process developments. On the other hand, the excess presence of carbon and hydrogen impurities could explain the small deviation in the stoichiometry of the films. It is possible that the excited state lifetime can be further increased by increasing the annealing temperature above 750 ◦ C. However, Serna et al. have reported surface damage and deterioration of

the photoluminescence performance when their Er-doped Al2 O3 samples were annealed above 850 ◦ C. 26 Nevertheless, the increase in the lifetime of the Er-ions is most likely the reason for the enhancement of the PL signal between the as-deposited samples and the samples annealed at 750 ◦ C as more Er-ions are allowed to decay radiatively rather than non-radiatively. The measured lifetimes of the samples annealed at 750 ◦ C are presented in Table 2. Finally, with the lifetimes known, we performed pump-power analysis of the 750 ◦ C sample set by varying the excitation intensity of the 532 nm laser in the range 0.62 − 3.8 · 104 W/cm2 and measuring the PL yield of the samples at 1.53 µm. Fig. 4b presents the experimental results, with points corresponding to the measured values and solid lines corresponding to the theoretical fits by Eq. S4 (see Supporting Information). In the fitting procedure, the co-operative up-conversion coefficient (C24 ), the fraction of quenched ions (q) and the PL collection efficiency were set as the fitting parameters for each sample. The collection efficiency included the responsivity of the photodetector, the losses in the optical components and the limited amount of photons that were collected. The lifetime of both the active and quenched Er-ions in state 4 I11/2 (τ3 ) and that of the quenched ions in state 4 I13/2 (τ2q ) were


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a)

b) As-deposited Annealed at 750 °C

1

PL Intensity (arb. unit)

PL Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

0.8 0.6 0.4 0.2 0

10

×10 20 0.6 % Er 1.5 % Er 3.5 % Er 4.1 % Er

8 6 4 2 0

0

10

20

30

0

1

2

3

4 2

Pump Intensity (W/cm ) ×10 4

Time (ms)

Figure 4: a) PL intensity of Sample 3 as a function of time in two cases: as-deposited (triangles) and annealed at 750 ◦ C (circles); b) PL intensity of the 750 ◦ C sample set at λ = 1.53 µm as a function of the pump intensity. In a) the points correspond to the experimental values of the timedecay measurements and the solid lines are the theoretical (nonlinear least-squares) fits according to Eq. S6, whereas in b) the points correspond to the experimental values of the pump-power measurements (converted into population scale) and the solid lines are the theoretical (nonlinear least-squares) fits according to Eq. S4. Table 2: Amplification properties of the Er:Al2 O3 samples annealed at 750 ◦ C. N0 (at. %)

τ2 (ms)

0.6 1.5 3.5 4.1

5.14 ± 0.045 5.15 ± 0.047 5.04 ± 0.035 4.89 ± 0.027

C24 (10−21 cm−3 s−1 ) 56.7 ± 14.2 111 ± 16.2 231 ± 16.4 260 ± 20.1

assumed to be 60 µs and 0.1 µs, respectively. 36 The exact pump rates (R17 ) were determined by measuring the absorption coefficient (σ17 ) of the 4 I15/2 →2 H11/2 transition in the samples for the 532 nm excitation, yielding 1.42±0.4·10−21 cm2 with only slight differences across the samples. The values for C24 and q that give the best (nonlinear least squares) fit for each sample are given in Table. 2. Table 2 also lists the values for the peak emission and absorption crosssections (σ21 , σ12 ) that were calculated by applying the theories originally introduced by Aull & Jenssen 37 and McCumber. 38 Our calculations show that σ12 ≈ 0.99σ21 , which agrees very well with the previous reports on Er-doped Al2 O3 . 1,7 In order to confirm the validity of the calculated values for C24 , the time-decay and pumppower measurements were compared, since the time-decay measurements can also be used to

q (%)

σ21 (10−20 cm2 )

σ12 (10−20 cm2 )

0.9 ± 0.4 1.6 ± 0.9 3.5 ± 1.1 19.2 ± 2.9

1.08 1.06 1.08 1.06

1.07 1.05 1.07 1.05

determine the value of C24 if N2a (0) is known. With I = 0.62 · 104 W/cm2 that was used in the time-decay measurements, one finds N2a = 2.97 · 1020 cm−3 from the pump-power measurements. Substituting this value for N2a (0) yields C24 N2a (0) = 70.19 → C24 = 2.36 · 10−19 cm−3 s−1 , which is very close to the value that was obtained from Sample 3 in the pumppower measurements. Similar agreement between the pump-power and time-decay measurements were also confirmed for the other samples. The relation between the total Er-concentration N0 and τ2a as well as the relation between N0 and C24 are shown in Figs. 5a and 5b, respectively. In Fig. 5a, an empirical law was applied, which predicts the evolution of the Erion lifetime τ2a as a function of the total Er-


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a)

b)

5.2

3

×10 -19

2.5

C 24 (cm -3 s-1 )

5.1 Er-concentration (at. %) 1

5

2

3

4 100 95 90

4.9

85 80

Fraction of active Er-ions (%)

τ2a (ms)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

1

2

1.5 1 0.5

75

4.8

2

0 3

4

0


1

2

3

4


Figure 5: a): N0 vs. τ2a and N0 vs. (1 − q) (inset); b) N0 vs. C24 . In a) the points correspond to the experimental values determined from the time-decay measurements and the solid line is the theoretical fit according to Eq. 1, whereas in b) the points correspond to the experimental values determined from the pump-power measurements and the solid line is the theoretical fit according to Eq. 2. concentration N0 in the presence of quenching: τ2a (N0 ) =

τ 2 p , N0 1+ NQ

Furthermore, the linear relation between N0 and C24 in Fig. 5b agrees with the experimental and theoretical reports found from the literature, e.g. by Agazzi et al., 36 according to whom the linear relation follows,

(1)

where τ2 is the lifetime of the sample in the absence of quenching (i.e. natural lifetime) and NQ is the critical quenching concentration. According to Eq. 1 and Fig. 5a, the lifetimes of the studied samples follow the empirical law (solid line) very well with p = 4 and critical quenching concentration of ∼ 8%. The deviation of p from 2, which describes the typical dipole-dipole interactions between the Er-ions, indicates the presence of dipolequadrupole, quadrupole-quadrupole or energytransfer involving more than two particles. 36 Although such processes are likely to occur in our samples due to them consisting of very high concentrations of Er-ions, we could not find an equivalent model in either the time-decay or pump-power measurements that would confirm their presence. Fig. 5a also demonstrates the relation between the total Er-concentration N0 and the fraction of active Er-ions (1 − q) as an inset figure. As the inset figure demonstrates, the relation between N0 and (1−q) follows that of N0 vs. τ2a very well.

C24 =

π2 p CDA CDD N0 , 3

(2)

where CDA and CDD are the donor-donor and donor-acceptor microparameters, and N0 is the total√Er-concentration. In our experiments, we find CDA CDD = 1.8 ± 0.3 · 10−41 cm6 s−1 . As a summary of this section, the comprehensive PL measurements of the fabricated samples demonstrate that we have been able to maximize the PL response of highly doped Er:Al2 O3 by delicately controlling the interionic distance between the Er-ions in the Al2 O3 host and also with suitable post-process annealing treatment. The optimization of our deposition technique allows us to fabricate Er-doped Al2 O3 with as high active Er-concentration as ∼ 3.5 % while maintaining the co-operative up-conversion at reasonable levels (∼ 10−19 cm−3 s−1 ) and at the same time, preserving the long excited state lifetime of the Er-ions. Incorporation of Erconcentration higher than 3.5 % resulted in rapid concentration quenching, which was con-


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firmed by the experimental quenched-ion rateequation model. Therefore, we believe that the sub-nano scale engineered Er:Al2 O3 samples fabricated in this work are most probably cluster-free with relatively weak interaction between the Er-ions as long the Er-concentration is kept below 3.5%. This shows significant advances than previously demonstrated methods, such as reactive co-sputtering, pulsed laser deposition or ion implantation that could incorporate less than 1 % of Er-ions into Al2 O3 before either PL quenching or rapid decrease in the lifetime occurred. 25,26,39 In fact, Van et al. have experimentally verified that up to 6% of Er-ions can be incorporated into Y2 O3 host with radically-enhanced ALD before the Er-Er interactions start to dominate. 30,40 Recently, the same group showed that by further optimizing the structure of their Er:Y2 O3 material, it is possible to fabricate cluster-free Er:Y2 O3 with Er concentration as high as 10 % before concentration quenching was observed. 28 This is also in line with our PL measurements of the as-deposited samples where the PL quenching could not be observed even at 4.1 % concentration levels. Furthermore, Van et al. also reported very similar interionic distances (∼ 4Å) between the Er-ions for optimized PL yield. On the other hand, Dingemans et al. studied the PL response of highlydoped Er:Al2 O3 fabricated with ALD. 41 They reported optimal PL signal at Er-concentration of ∼ 3% with PL quenching at higher concentrations, which agrees very well with the results presented here. However, neither of these groups reported the lifetimes or the cooperative up-conversion coefficients of their Erdoped material, which makes it impossible to predict the efficiency of their material as an active device for, e.g. integrated waveguides. In addition, both groups reported extremely slow deposition rates for their Er-doped material (i.e. up to tens of seconds were used for half-cycle pulsing or purging as compared to few seconds in our case). Therefore, the methods demonstrated by Van et al. and Dingemans et al. are very impractical when it comes to waveguide applications that require few hundred nanometers of the active medium.

In this work, we not only demonstrate that a high concentration of active Er-ions can be incorporated into Al2 O3 with ALD, but also show that the long excited state lifetime of the Er-ions can be maintained in this active medium with relatively low co-operative upconversion coefficient for efficient amplifier operation. Thus, we present an optimized, efficient and practical method of fabricating highly doped Er:Al2 O3 gain medium for active integrated waveguide devices.

Conclusions In the present work, we fabricated highly doped Er:Al2 O3 thin films with ALD by sequentially depositing Er2 O3 and Al2 O3 on top of silicon substrate. In order to minimize the energy transfer between Er-ions, sufficient lateral distance between the Er-ions was ensured by choosing large Er(thd)3 precursor for the fabrication of Er2 O3 . Interionic distance in vertical direction was controlled by introducing subsequent layers of Al2 O3 between Er2 O3 -layers. PL response of the fabricated samples confirmed that our technique is capable of incorporating up to ∼ 3.5% of Er-ions into Al2 O3 before rapid concentration quenching occurs. Time-decay and pump-power measurements confirmed that the long lifetime (∼5 ms) of the first excited state in the Er-ions is preserved with relatively low co-operative up-conversion coefficient (C24 ∼ 10−19 cm−3 s−1 ) even at such high concentration levels. As a conclusion, this work demonstrates the exceptionality of ALD as the fabrication technique to deposit highly doped Er:Al2 O3 with excellent amplifier properties for integrated photonics. Supporting Information Available: Modeling the photoluminescence response of highly-doped Er:Al2 O3 using the quenchedion rate-equation analysis. This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgement The authors acknowledge the financial support from Aalto ELEC doctoral school, TEKES - the Finnish Funding


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Agency for Innovation (FiDiPro: NP-Nano and OPEC), Academy of Finland (grants: 276376, 284548, 285972), the European Union’s Seventh Framework Programme (REA grant agreement No. 631610) and thank Micronova Nanofabrication Centre for providing the facilities. Tero Pilvi of PICOSUNTM is acknowledged for the help with the ALD process development for Er2 O3 .

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(7) Wörhoff, K.; Bradley, J. D. B.; Member, S.; Ay, F.; Geskus, D.; Blauwendraat, T. P.; Pollnau, M. Reliable LowCost Fabrication of Low-Loss Al2 O3 :Er3+ Waveguides with 5.4-dB Optical Gain. IEEE J. Quantum Electron. 2009, 45, 454–461. (8) Becker, P. C.; Olsson, N. A.; Simpson, J. R. Erbium-doped fiber amplifiers : fundamentals and technology; Academic Press: San Diego, 1999.

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tegrated with a 980/1530 nm WDM coupler. Electron. Lett. 1994, 30, 856–857.

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(16) Serna, R.; Ballesteros, J. M.; De Castro, M. J.; Solis, J.; Afonso, C. N. Optically active Er-Yb doped glass films prepared by pulsed laser deposition. J. Appl. Phys. 1998, 84, 2352–2354.

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(17) Shmulovich, J. Integrated planar waveguide amplifier with 15 dB net gain at 1550 nm. Optical Fiber Communication Conference. 1999. (18) Orignac, X.; Barbier, D.; Du, X. M.; Almeida, R. M.; McCarthy, O.; Yeatman, E. Sol–gel silica/titania-on-silicon Er/Yb-doped waveguides for optical amplification at 1.5 µm. Opt. Mater. 1999, 12, 1–18.

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(19) Le Quang, A. Q.; Hierle, R.; Zyss, J.; Ledoux, I.; Cusmai, G.; Costa, R.; Barberis, A.; Pietralunga, S. M. Demonstration of net gain at 1550nm in an erbiumdoped polymersingle mode rib waveguide. Appl. Phys. Lett. 2006, 89, 141124.

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(32) van Hemmen, J. L.; Heil, S. B. S.; Klootwijk, J. H.; Roozeboom, F.; Hodson, C. J.; van de Sanden, M. C. M.; Kessels, W. M. M. Plasma and Thermal ALD of Al2 O3 in a Commercial 200 mm ALD Reactor. J. Electrochem. Soc. 2007, 154, G165–G169.

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(41) Dingemans, G.; Clark, A.; Van Delft, J. A.; Van De Sanden, M. C. M.; Kessels, W. M. M. Er3+ and Si luminescence of atomic layer deposited Er-doped Al2 O3 thin films on Si(100). J. Appl. Phys. 2011, 109, 113107.

(34) Serna, R.; de Castro, M. J.; Chaos, J. a.; Afonso, C. N.; Vickridge, I. The role of Er3+ -Er3+ separation on the luminescence of Er–doped Al2 O3 films prepared by pulsed laser deposition. Appl. Phys. Lett. 1999, 75, 4073–4075. (35) Lazarouk, S. K.; Mudryi, A. V.; Borisenko, V. E. Room-temperature formation of erbium-related luminescent centers in anodic alumina. Appl. Phys. Lett. 1998, 73, 2272–2274. (36) Agazzi, L.; Wörhoff, K.; Pollnau, M. Energy-Transfer-Upconversion Models, Their Applicability and Breakdown in the Presence of Spectroscopically Distinct Ion Classes: A Case Study in Amorphous Al2 O3 :Er3+ . J. Phys. Chem. C 2013, 117, 6759–6776. (37) Aull, B.; Jenssen, H. Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections. IEEE J. Quantum Electron. 1982, 18, 925–930. (38) McCumber, D. E. Einstein Relations Connecting Broadband Emission and Absorption Spectra. Phys. Rev. 1964, 136, A954– A957. (39) Kik, P. G. Energy transfer in erbium doped optical waveguides based on silicon. Doctoral dissertation, 2000. ACS Paragon Plus Environment

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Atomic layer engineering of Er-ion distribution in highly doped Er:Al2O3 for photoluminescence enhancement John Rönn,∗,† Lasse Karvonen,† Christoffer Kauppinen,† Alexander Pyymaki Perros,† Nasser Peyghambarian,‡,†,¶ Harri Lipsanen,† Antti Säynätjoki,†,¶ and Zhipei Sun† †Department of Micro- and Nanosciences, Aalto University, Espoo, Finland ‡College of Optical Sciences, University of Arizona, Tucson, USA ¶Institute of Photonics, University of Eastern Finland, Joensuu, Finland E-mail: [email protected];[email protected] Phone: +358 (0)503549262

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