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Abstract
Thin layers of nickel (Ni) and palladium (Pd) are used to form Schottky barrier photodiodes on silicon–on–insulator optical rib waveguides defined by the local oxidation of silicon technique. Optical loss and attenuation due to the metallic layers on the waveguides are estimated by simulations followed by experimental verification. Loss increases with increasing metal thickness until a plateau is reached at a thickness of approximately 50 nm. Higher optical loss is observed for the transverse magnetic mode compared to that for the transverse electric mode. The dark current density of all the devices is less than 10−6Acm−2 at 1 V reverse bias. The TM mode responsivity is 4.7 mA/W and 0.33 mA/W for 0.5 mm long Ni/nSi and Pd/nSi at 1310 nm wavelength, respectively. This work demonstrates great potential for simple sub-bandgap photodetectors for silicon photonics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There has been great interest in the integration of electronic function with planar optical waveguides in the silicon-on-insulator (SOI) platform [1]. To fully realize the potential of integrating electronics with optics efficient on-chip SOI sub-bandgap photodetectors [2] in the 1310 – 1550 nm free space wavelength range are required. Various techniques for forming such photodetectors have been reported. One of the simplest techniques involves the deposition of a metal layer over a waveguide, forming a metal-semiconductor Schottky barrier diode photodetector [3–5]. With an appropriate choice of metal and hence barrier height good sub-bandgap photoresponse can be obtained with relatively low dark current leakage. Here we focus only on palladium (Pd) and nickel (Ni) barrier layers, which are suitable for the 1310 - 1550 nm wavelength range [4]. Although other metals, such as platinum (Pt) and titanium (Ti), may also be suitable for this telecommunication wavelength range, the Schottky barrier height of the metal/Si are either too large or too small, offering lower photoresponse or higher reverse leakage respectively [6–8]. The optical absorption by the Schottky metallic layer is the main contributor to the photocurrent generation, but it also causes optical loss and attenuation in the metal-clad optical waveguide. There have been a few studies on Schottky barrier photodetector absorption through numerical modelling and experiments [9–11]. Optical loss of a conventional SOI waveguide has been investigated by Rickman et al [12] and Dumon et al [13], where a comprehensive analysis on the waveguide mode and propagation characteristics was provided. Studies on the scattering losses due to waveguides sidewall roughness were reported in [14–16]. All of the reported rib waveguides were fabricated by using deep UV lithography and reactive ion etching. We believe that the local oxidation of silicon (LOCOS) technique [17–19], where selective oxidation is used to define a rib or ridge waveguide surrounded by thermally-grown oxide, has great potential to produce the ultralow loss SOI waveguides due to the inherent smoothness of the oxidized waveguide sidewall. In this work, we demonstrate the integration of Pd/n-Si and Ni/n-Si Schottky barrier photodiodes with low-loss LOCOS rib SOI waveguides in Fig. 1a
Fig. 1 SEM image of a LOCOS SOI waveguide (a) and schematic of 3D waveguide photodetector structure (b).
2. Optical loss simulation
The optical loss due to the metallic layer absorption and transmission was estimated using the FIMMWAVE (Version 6.0.1) Film Mode Matching (FMM) solver and Lumerical mode solutions. The FMM solver is a semi-analytical, fully vectorial waveguide solver based on the film mode matching method [20]. It is generally for waveguides which have large regions of uniform refractive index. It can accurately model lossy, metallic, leaky, or radiating structures. Since the loss in a 1 mm LOCOS Si waveguide is known to be quite low [18], the primary contributor to loss estimated in the simulations is the presence of the metal layer. The refractive indices of metals are complex, where the imaginary part describes the optical absorption. The optical properties of Ni and Pd are given in Table 1
Table 1. Optical Properties of Ni and Pd at 1310 nm and 1550 nm Wavelength
Here n and k are the real and imaginary part of refractive index, respectively; α is the absorption coefficient. α = 4πk /λ. Power loss can be calculated based on Bouguer’s Law P = P0e-ax where P is the optical power at depth x, P0 is the original input power and x is the metal layer thickness [22]. The waveguide structure was simulated with the mixed-geometry waveguide (FWG) interface in which the slanted ridge sidewalls typically formed by the LOCOS process can be properly approximated. The waveguide rib was set to 4 µm in width and 0.5 µm in height, topped with a Schottky metal layer with thickness of 10 – 100 nm. The waveguide core size chosen is larger than that of many current wire and ridge waveguides, but is compatible with simple, inexpensive optical lithography tools and in particular avoids the need for e-beam or deep UV lithography. Furthermore, recently there has been more interest from industry (e.g., Rockley Photonics) in working with larger waveguides for ease of light coupling and fabrication. According to the simulations, this rib structure supports only the fundamental transverse electric (TE) and transverse magnetic (TM) modes. The electric field profiles of the fundamental modes are shown in Fig. 2
Fig. 2 The Ex profile of the fundamental TE mode in a rib SOI waveguide. The target rib width wr is 4 µm and rib height hr is 0.5 µm, at the top of the rib there is a 30 nm Ni metal layer.
The optical loss of the SOI waveguide as a function of the Schottky metal thickness is shown in Fig. 3 (a) and (b)
Fig. 3 Simulated optical loss of the SOI waveguide (wr = 4 µm, hr = 0.5 µm, H = 3.4 µm) under a TE mode (a) and a TM mode (b) as a function of the Schottky metal thickness for various free-space wavelengths. Two metals are used on top of the rib waveguide: Pd and Ni.
Fig. 4 Simulated optical loss of the SOI waveguide (wr = 4 µm, hr = 0.5 µm, H = 3.4 µm) versus free space wavelength for TE and TM polarization, with a 10 nm metal cladding layer (Ni or Pd).
3. Fabrication and measurements
Test devices were fabricated on SOI wafers produced using the SmartCut technique with Si film thickness of 3.4 µm and buried oxide thickness of 0.4 µm. The Si active layer was implanted with phosphorus and annealed to provide an active doping level of approximately 1016 cm−3. Following the LOCOS technique reported in [17], a 20 nm pad oxide was first grown thermally, then a 50 nm Si3N4 etch mask layer was formed by chemical vapor deposition. The nitride and pad oxide were patterned to define long, straight waveguides using optical contact lithography and plasma etching. The waveguide ribs were formed by thermal oxidation, giving a rib height (hr) of 500 nm, after which the top nitride and pad oxide were removed to expose bare silicon at the top of the rib. The rib sidewalls are clad with thermal oxide. The target rib width (wr) here was 4 µm, but our process has been successful in forming ridge waveguides with widths as small as 700 nm [24]. A 10 nm Ni or Pd metal layer was deposited over the top of the waveguides using electron beam vacuum evaporation, followed by patterning using lift-off lithography. Samples were flash etched in 1% hydrofluoric acid immediately prior to metallization to minimize the thickness of native oxide. Ohmic contacts were made using large-area Al pads, connecting to the top of waveguide ribs running close to and parallel to the active waveguides. Waveguide ends were cleaved to allow end-fire coupling.
The schematic of optical measurement setup is demonstrated in Fig. 5
. TE or TM polarized light at 1310 nm or 1550 nm wavelength was injected into the waveguides from an Agilent 8164A source through a tapered optical fiber. The fiber was mounted on a translation stage which provided x, y, z positioning as well as the ability to rotate along the horizontal x-y plane and the vertical plane. The output was collected by a second tapered fiber and measured by an Exfo PM-1600 High-speed power meter.The total loss measured in our experiments includes the SOI waveguide propagation loss, fiber-to-chip coupling loss due to the modal mismatch, facet reflection due to index mismatch, Schottky metal absorption loss, as well as the measurement system loss. Given that previous work has shown that the propagation loss of LOCOS-defined optical waveguides with the dimensions used here can be lower than 1 dB/cm [18] in the absence of a metallic overlayer, Table 2
Table 2. Optical Loss of SOI Waveguide Due to a 10 nm Ni and Pd Metal Layer
Table 3. Electrical and Optical Performance of Schottky Diodes
Optical responsivity was measured at wavelengths of 1310 nm and 1550 nm. The Ni/nSi diode had the best optical performance: the responsivity is 10 times stronger than that of the Pd/nSi diode. However, the low barrier height of these devices gives high dark current. The high barrier height of the Pd/nSi diode gives low dark current but the optical response is relatively poor, particularly at 1550 nm wavelength.
Table 3 shows that the responsivity rolls off quite rapidly over the wavelength range from 1310 to 1550 nm, with measurable response at 1550 nm. This is due to the decreasing photo responsivity at longer wavelengths, especially when the operating wavelength is just about the cut-off wavelength (λ = 1.24/Փb) of 1570 nm [25]. Although the results reported in our work are not better than the previous experimental results reported in [2–4], we believe that we can improve the electrical performance of our devices by increasing Schottky metal thickness and photodetector length. The Ni/nSi photodetector was evaluated under both TE and TM polarizations. The optical response with TE polarization is approximately 60% of the value obtained from the TM polarization, because the TM and TE mode have different field distribution inside and outside of the waveguiding core. The TM mode has more evanescent field extending into the top layer of the waveguide ridge [26], exciting more surface plasmons at the metal/nSi interface. Therefore, the Schottky metal film above the ridge absorbs more light power and generates a larger photo current with the TM polarization. This interpretation was confirmed by FIMMWAVE simulations and optical loss measurements mentioned above.
4. Conclusions
In conclusion, two types of Schottky diodes - Ni/nSi and Pd/nSi - were successfully fabricated on LOCOS-defined waveguides. We presented our investigation of the electrical and optical characteristics of these devices. The dark current density is 10−6 A/cm2 and 2 × 10−8 A/cm2, and the photo responsivity for TM at 1310 nm is 4.7 mA/W and 0.33 mA/W for Ni/nSi and Pd/nSi diode, respectively. The photo responsivity decreased as the wavelength increased but still gave measurable signals at 1550 nm wavelength. The optical loss caused by the presence of Pd and Ni metal overlayer in these waveguide photodetectors was examined here through both simulations and experiments. Losses are significantly higher for the TM mode than for the TE mode, as expected. Simulated loss increases roughly linearly with increasing metal thickness until a plateau is reached at a thickness of about 10 nm (for TE) or 20 nm (for TM). The plateau loss is approximately 1 dB/mm for TE and 2 dB/mm for TM. Loss increases slightly with increasing wavelength in the range between 1310 and 1550 nm. Measured losses for fabricated devices with 10 nm thick Pd and Ni metal layers were slightly higher than those predicted by simulations, but showed the same general trends. Although the dimension of the optical waveguides in the experiments are yet to be optimized, this work has successfully demonstrated through simulation and experiment that, with an appropriate choice of barrier metal, Schottky photodiodes integrated with LOCOS-defined waveguides can provide simple yet effective sub-bandgap photodetectors for SOI photonics.
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