Si-Nanocrystals-Embedded SiOx Film on Silicon-on-Insulator Substrate
In this study, we demonstrated the light enhancement from a Si-nanocrystals-embedded SiOx film on a silicon-on-insulator (SOI) substrate in visible light range. The light emission from the annealed SiOx film is one order stronger than emission from non-annealed SiOx film. Compared with the SiOx film on a Si substrate, two-fold enhancement in light emission from the SiOx film on SOI substrate was also observed. The enhancement was attributed to better vertical confinement of optical field in the SiOx film on SOI substrate.
Introduction
In recent years, silicon photonics has received a great deal of attention because of its compatibility with existing electronic circuits. Si-based light sources with the mainstream complementary metal-oxide semiconductor (CMOS) technology have been widely studied and developed [1,7]. Compared to III-V semiconductor emitter, silicon light sources have better integration ability and low fabrication cost. Therefore, Si-based materials could be the potential candidate for future optoelectronic integrated circuits [2-7]. However, bulk silicon has lower emission efficiency due to its indirect transition characteristics. To achieve light emission from silicon at room temperature, there are lot of reports for low-dimensional silicon systems, such as porous silicon [8,9], silicon nanocrystals[10], and superlattices[11].
Recently, light emission from silicon-rich SiNx [12, 13] (SRN) and SiOx films [14, 15]
with 3–5 nm Si nanocrystals has been widely studied. The luminescence is attributed to confine exciton recombination in the Si-nanocrystals owing to the emission wavelength is related to Si-nanocrystals size [15, 16]. Several different techniques which are produced Si nanocrystals in SiOx were investigated, namely, ion implantation, chemical vapor deposition, sputtering, and laser ablation [14, 17-19]. In this report, light emission from a
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Si-nanocrystals-embeded SiOx film on a silicon-on-insulator (SOI) substrate was investigated. A strong light enhancement from the SiOx film on SOI substrate was observed due to the improvement in vertical optical confinement.
Fabrication process
This SOI wafer was prepared with horizontal furnace system. A 2.3μm thick SiO2 layer was first grown on a Si substrate followed by the growth of a 250 nm thick poly-Si layer on the top of the SiO2 layer. A 360 nm thick Si-rich SiOx film was grown on the SOI substrate by the plasma-enhanced chemical vapor deposition (PECVD) system with parameters in the previous works [14, 15]. The SiOx film was annealed in a quartz furnace with N2 gas at 1100 °C for 90 min to precipitate Si nanocrystals. This anneal step is important to obtain strong emission from the Si-rich SiOx film. We also fabricated the SiOx film on a bare Si substrate as the reference. The illustrations of the SiOx/Si and SiOx/SOI structures are shown in Fig. 1(a). Fig. 1(b) shows a SEM cross-sectional image of the SiOx/SOI structure.
Figure 1 (a) Schematic structure of SiOx film on SOI and Si substrates.
(b) The SEM image of the SiOx/SOI sample from cross-section view.
(b)
SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
Si substrate
SiOx～ 360nm (gain material)
Si substrate
SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
(a)
SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
Si substrate
SiOx～ 360nm (gain material)
Si substrate
SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
Si substrate
SiO2 ~ 2.3μm Poly-Si ～ 250nm
SOI SiOx～ 360nm (gain material)
(a)
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Results and discussion
To characterize the light emission from the Si-nanocrystals-embeded SiOx film on SOI and Si substrates, the devices were optically pumped by using a CW He-Cd laser at 325 nm with an incident power of 40mW and a pumped spot size of 30μm. Fig. 2 shows the photography of a SiOx/SOI structure. Fig. 3 shows the measured emission spectra from the different devices. The black solid curve in the Fig. 3 is the emission spectrum from a SiOx film without the annealing procedure. It shows a very low emission from the as-grown SiOx film. The blue-dashed and red-dotted curves are the emission spectra from the SiOx films on Si and SOI substrates, respectively. The light emission of SiOx film is enhanced more than ten times after annealing step. This is because the self-aggregation of silicon nanocrystals in the SiOx film after annealing process at 1100 °C. The optical band gap of this Si-rich SiOx films was also red-shifted due to increase of Si-Si bonding states, which was observed from the spectra (red and blue) from the annealed SiOx/Si and SiOx/SOI devices. The emission wavelength is strongly related to the size of Si-nanocrystals [20].
By comparing the blue and red spectra, the light emission from the SiOx/SOI structure is two times higher than the emission from the SiOx/Si structure under the same pumping conditions. The higher emission of SiOx film on SOI substrate is attributed to the better optical confinement due to lager index contrast of the structure. In order to understand the details of higher emission from the SiOx/SOI structure, we study optical field in the structure by using transfer matrix method. The distributions of the refractive indices of SiOx/Si and SiOx/SOI structures are shown with red curves in Fig. 4(a) and (b). The black curves in Fig. 4(a) and (b) are the electric field of fundamental modes in the vertical direction for the two structures. The mode distribution of SiOx/SOI structure is more concentrated around the SiOx gain layer. The effective indices of the fundamental modes are 3.88 and 3.82 for SiOx/Si and SiOx/SOI structures. According to the calculated results, we also estimated the fraction of electric field in the SiOx layer. The estimated fraction values of the SiOx/SOI and SiOx/Si structures are 1.3% and 0.02%, respectively. As a consequence of calculation, the optical field in the SiOx/SOI structure would be more confined than field in SiOx/Si structure. In experiment, this improvement in optical confinement gave a two-fold enhancement in emission spectrum intensity. The total
optical power from a light emitter can be estimated by integrating spectrum over all wavelengths (i.e. P=
∫
P(λ)dλ). The optical power from the SiOx/SOI device is 1.7 time higher than the power from the SiOx/Si device.In Fig. 3, the emission spectrum from the SiOx film on SOI substrate shows the small oscillation behavior, which was not observed in the spectrum from SiOx/Si structure. This oscillation is attributed to Fabry-Perot resonance of the pumping signal in the SiO2 layer.
The similar interference fringes were also observed in the LEDs structures having the flat surface [21-22]. Those different light emission resonant modes can be analyzed by the following equation:
(1)
Where λ is wavelength of the emission light, n and d are the index and thickness of the SiO2 layer, which are 1.7 and 2.3μm, respectively. Δλ is the free spectral range of the Fabry-Perot oscillation. The estimated free spectral range Δλ is about 50nm, which agrees to the measured results. This oscillation could be reduced by increasing the thickness of the Si layer to reduce emission penetrating into SiO2 layer or introducing surface texture at the Si-SiO2 interface.
nd 2
λ
2λ
= Δ
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Figure 2 The photography of the SiOx/Si/SiO2 device during the experiment.
Figure 3 The emission spectra of SiOx film on SOI and Si substrate within the visible range after annealing procedure. The Fabry-Perot interference fringes were observed in the spectrum of SiOx/SOI structure.
300 400 500 600 700 800 900 0
4000 8000 12000 16000 20000
Intensity (a.u. )
Wavelength (nm)
as-grownth SiOx SiOx/SOI
SiOx/Si
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Figure 4. The distribution of the material refractive indices (red) and the fundamental mode along the vertical direction of (a) SiOx/SOI and (b) SiOx/Si structures.
Conclusions
In summary, we had demonstrated the visible light enhancement from the PECVD grown Si-nanocrystals-embeded SiOx film on SOI and Si substrates. The emission is enhanced by ten times with the annealing process, and two times with better optical confinement in SOI structure. We employed the transfer matrix method to verify that the optical confinement of SiOx/SOI structure is much better than SiOx/Si structure. It leads to stronger emission from SiOx/SOI structure. The oscillation in emission spectrum of SiOx/SOI structure is also verified to be Fabry-Perot oscillation in the SiOx layer.
Because of the strong light enhancement from the Si-rich SiOx/SOI device, the material has a high potential to be Si-based light sources in the future applications.
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Appendix II
Fabrication Technique – Electron-Beam Lithography
The EBL is a technique using electron beam to generate patterns on a surface with a resolution limited by De Broglie relationship (λ < 0.1 nm for 10-50 KeV electrons), which is far smaller than the light diffraction limitation. Therefore, it can beat the diffraction limit of light to create a pattern which only has a few nanometers line-width without any mask. The first EBL machine, based on SEM system, was developed in the 1960s. The EBL system usually consists of an electron gun for generating electron beam, a beam blanker for controlling the electron beam, electron lenses for focusing the electron beam, a stage and a computer control system as shown in Figure 2.10. Figure 2.11, Figure 2.12, and Figure 2.13 show the photography of the PECVD, the ICP-RIE (Oxford Plasmalab System 100), and the ICP-RIE (SAMCO RIE-101PH) of two etching steps, respectively. And the detail recipe of each fabrication process is shown in the following description.
E-beam Lithography System (JEOL JSM-6500) Spin coating use PMMA (A5)
First step: 1000 rpm for 10sec.
Second step: 3500 rpm for 25sec.
Hard bake: hot plate 180℃, 90sec.
Exposure:
Beam voltage: 25KeV Dosage: 1.4~1.7 (point does)
Development: MIBK: IPA(1:3) 70sec.
Fixing: IPA 40sec.
PECVD (SAMCO PD220) Si3N4 film deposition:
SiH4/Ar: 20sccm NH3:10sccm N2:490sccm
Temperature: 300℃
150
RF Power: 35W Pressure: 100Pa
Time: 31 min for 300nm thick Si3N4
SiO2 film deposition:
SiH4/Ar: 25sccm N2O:500sccm N2:250sccm
Temperature: 250℃
RF Power: 35W Pressure: 120Pa
Time: 1 min for 20nm thick SiO2
ICP-RIE (Oxford Plasmalab System 100) Si3N4 film etching:
Ar/O2: 5sccm CHF3: 50sccm RF Power: 150W Pressure: 7.5x10-9Torr Temperature: 20℃
Time: 3min. 35sec. to etch 300nm thick Si3N4 film ICP-RIE (SAMCO RIE-101PH)
GaN film etching:
Cl2: 25sccm Ar: 10sccm ICP Power: 200W Bias Power: 200W Pressure: 0.33Pa
Time: 55sec. to etch 500nm thick GaN film
Figure 1 (a) The typical schematic diagram of EBL system(JEOL JSM6500). (b) Schematic electron gun of EBL system.
(a)
(b) (a)
(b)
152
Figure 2 PECVD systems.
Figure 3 ICP-RIE (Oxford Plasmalab System 100)
154
Figure 4 ICP-RIE (SAMCO RIE-101PH)