- Nano Express
- Open Access
Thickness-dependent optimization of Er3+ light emission from silicon-rich silicon oxide thin films
© Cueff et al; licensee Springer. 2011
Received: 25 January 2011
Accepted: 25 May 2011
Published: 25 May 2011
This study investigates the influence of the film thickness on the silicon-excess-mediated sensitization of Erbium ions in Si-rich silica. The Er3+ photoluminescence at 1.5 μm, normalized to the film thickness, was found five times larger for films 1 μm-thick than that from 50-nm-thick films intended for electrically driven devices. The origin of this difference is shared by changes in the local density of optical states and depth-dependent interferences, and by limited formation of Si-based sensitizers in "thin" films, probably because of the prevailing high stress. More Si excess has significantly increased the emission from "thin" films, up to ten times. This paves the way to the realization of highly efficient electrically excited devices.
The realization of efficient Si-based optical emitters for photonics is one of the most challenging objectives for the semiconductor community . Such a purpose is confronted to the indirect band gap of bulk silicon which makes difficult the light emission from Si, and then presents a major obstacle to full photonic-electronic integration. However, the indirect sensitization of emission from erbium ions, via Si nanoclusters (Si-nc), in the technologically important 1.5-μm spectral region is a promising approach that has received significant attention. Such a sensitizing effect of Si-ncs increases the effective excitation cross section of Er by 103-104 over a broad band in Si-rich silicon oxide (SRSO) systems . This leads to the observation of enhanced Er photoluminescence (PL) and electroluminescence in the standard telecommunications wavelength band around 1.54 μm [2, 3]. Depending on the targeted application, the thickness of the active layer can vary over a large range, from a micrometer-scale for planar waveguide amplifiers  to a few tens of nanometers for electrically driven LEDs  or slot waveguides . According to recent studies, layer thickness was shown to influence the nucleation and growth of Si-ncs [6–8], as well as the effective intensity of the pump beam  and the local density of optical states (LDOS) [10, 11]. This thickness dependence is crucial since each application requiring a given thickness may necessitate a specific optimization of the material.
In this paper, we investigate the impact of layer thickness on the optical properties of SRSO:Er thin films. The results demonstrate that the photoluminescence in very thin layers is hindered by some thinness-related limiting factors. To overcome this drawback of thin layer, more Si excess was gradually incorporated until a level of Er emission that was found surprisingly higher than that observed in optimized micrometer-thick layers.
The SRSO films doped with Er were grown onto a p-type, 250-μm thick, (100) silicon wafer, by magnetron co-sputtering of three confocal cathodes (SiO2, Si and Er2O3) under a plasma of pure Argon at a pressure of 2 mTorr. The power densities applied on the three confocal targets were kept constant, while the deposition was performed at two temperatures T d, room temperature (RT) and 500°C, for various durations between 20 min and 10 h. To examine the influence of Si excess for a set of thin films of about 50 nm in thickness, the power density on the Si target was subsequently increased. The thickness and refractive index n were measured by spectroscopic ellipsometry for films thinner than 500 nm and by m-lines techniques for films exceeding 500 nm in thickness. The thickness shows a linear variation with the deposition duration. The PL spectra were recorded using the non-resonant 476-nm excitation wavelength in order to ensure that Er3+ ions are only excited through the sensitizers. The samples were excited with 45° incident spot of approximately 3 mm2 with a power of 180 mW, i.e., a power density of 0.06 W/mm2. The Er content was obtained by time-of-flight secondary ion mass spectroscopy technique after calibration by a reference SRSO:Er sample containing a known Er concentration. The erbium concentration was found nearly constant for all samples at about 3 × 1020 at. cm-3. The Si excess was evaluated by two methods: X-ray photoelectron spectroscopy (XPS) exploring beyond 100-nm depth (or total thickness for thinner films) in different places, and Fourier transform infrared (FTIR) spectroscopy with a spot covering a large area of the sample. Transmission electron microscopy (TEM) observations were performed using a JEOL 2010F operated at 200 kV.
with n(σox) the refractive index for a given thickness, n 0 the refractive index for relaxed or "bulk" SiO2 (1.458) and Δn/Δσox = 9.10-12 Pa-1, taken from Ref. . The inset in Figure 2, shows a pronounced increase of n for a range of our thin films (<150 nm) for both matrix (SiO2 and SRSO) and is similar to that reported in Ref. , hence attesting of a thickness-dependent stress. The stress difference can be estimated to 4-6 GPa between the thinnest and thickest films. The main origin of this internal stress arises from the misfit between the substrate and the film. Its progressive increase when the films' thickness is reduced seems to inhibit the agglomeration of Si.
To overcome these limitations, we have gradually raised the Si excess in approximately 50-nm-thick films, with the objective of increasing the number of Si-based sensitizers. We show in Figure 5b the evolutions of I PL containing approximately 7.5 at.% Si excess (circles) as a function of the film thickness and I PL of thin films (approximately 50 nm) with different Si excess (squares) for the samples processed using optimized conditions (T d = 500°C, T a = 900°C, see Figure 3).
We plot in the inset of Figure 5b the evolutions of I PL in function of the Si excess for the 50-nm-thick films. The I PL optimum is reached for about 14 at.%, before decreasing for higher Si contents. In parallel, we observe a gradual and systematic decrease of the lifetime of Er emission, from nearly 1.8 ms to about 1 ms (not shown). This reflects the creation of new non-radiative decay channels , which should attenuate the Er PL. For Si excess lower than 14 at.%, such an attenuation is somehow dominated by the increase of excitation of Er3+ ions through more sensitizers. Beyond 14 at.%, the new non-radiative decay channels start to dominate, leading to the observed decline of Er PL . The Er PL peak intensity is ten times that of the similar thin film containing 7.5 at.% excess Si, and five times that observed for optimized thick samples containing 7.5 at.% excess Si (see corresponding symbols at the left part of Figure 5). Such an optimisation of the Si excess for 1-μm-thick samples was made earlier . The optimum Si excess in these 50-nm-thick films is almost twice the excess incorporated in the best thin layers studied so far by our team . This offers the double advantage of minimizing the limiting factors present in thin films, and favoring the transport of electrically injected carriers. In addition, the proportion of Er ions coupled to sensitizers is likely to be significantly improved, allowing one to expect a fraction of inverted Er much higher than the reported 20% .
In summary, the influence of layer thickness on the photoluminescence of Er ions has been investigated for SRSO:Er layers. It was shown that thinness-related effects decrease the PL for thin films by a factor of 5. These effects are mainly due to three origins: (1) high stress prevailing in thin films that inhibits the formation of Si nanoclusters, (2) changes in LDOS, and (3) changes in the pumping rates. To minimize the thinness-related limitations in thin films, the amount of Si excess was gradually increased until reaching an Er PL intensity one order of magnitude higher than that recorded earlier for similar thin samples. Such a route appears very promising for the improvement of electrically driven high-performance Si-based light sources.
The authors would like to thank Dr. A. J. Kenyon (University College London) and Dr. R. J. Walters (FOM institute Amsterdam) for fruitful discussions.
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