Graphene/Al₂O₃/Si Schottky diode with integrated waveguide on a silicon-on-insulator wafer

Abstract

Waveguide-integrated graphene photodiodes are on-chip optoelectronic devices with promising applications in telecommunications. Here, we present the electrical properties of a heterostructure consisting of multilayer graphene (MLGr) over a Si waveguide covered by an ultrathin Al₂O₃ layer. The waveguide is fabricated by etching a silicon-on-insulator (SOI) substrate with 220 nm Si and 1.5 μm buried oxide. The 5 nm-thick Al₂O₃ film is deposited by atomic layer deposition (ALD), while graphene, synthesized on copper by chemical vapor deposition (CVD), is transferred onto the Al₂O₃/Si rib by a wet transfer method. The MLGr/Al₂O₃/Si rib forms a Schottky structure with rectifying current–voltage characteristics, which are examined using the thermionic emission theory and Norde’s method. A Schottky barrier height ${\Phi }_{{\text{B}}} = 0.79\mathrm{ eV}$, an ideality factor n = 26, and a series resistance ${{\text{R}}}_{{\text{S}}} = 11.6\mathrm{ M\Omega }$ are obtained. The device is promising for operation at the optical fiber communication wavelength of 1550 nm.

Appendix – Fabrication and experimental details

First, the SOI wafers were cleaned with standard cleaning methods; the cleaning process consisted of ultrasonic cleaning in acetone and isopropyl alcohol (IPA), and the substrates were then dried with a nitrogen gun. After that, oxygen plasma was applied in a reactive ion etching (RIE) for 10 s to remove organic residue on the surfaces. Oxygen plasma parameters were: O₂ flow, 20 sccm; RF power, 50 W; chamber pressure, 37.5 mTorr; and room temperature, 20 °C. Considering the structures of the devices with narrow optical waveguides, a negative resist (ma-N 2401) was used for the photolithography processes, in which the exposed regions become insoluble and resistant to developers. The resist was spun at 2000 rpm for 30 s by a spin coater and baked on a hot plate for 90 s at 90 °C. The resist thickness was obtained as 50 nm. The same process is repeated on top of the first resist and a total e-beam resist thickness of ~ 100 nm was obtained. The resist was exposed using an e-beam lithography system with an intensity of 500 μC/cm². The pattern was developed directly by a TMAH-based developer (726 MIF) for 10 s. The pattern after development is shown in Fig. 3. Before etching, for the cleaning of resist residues that may be caused by the developed process, oxygen plasma was applied for 5 s with the oxygen plasma parameters given above. The gas mixture of C₄F₈/SF₆/Ar (50/25/20 sccm) was used in the silicon etching with RIE (ICP power, 1300 W; RF power, 50 W; temperature, 20 °C; pressure, 12 mTorr). With this recipe, approximately 240 nm silicon per minute is etched anisotropically, while the beam that acts as a mask and the resist that determines the frame is etched at the rate of ~ 90 nm per minute. After the etching process, the residue of 10 nm is cleaned in hot acetone. Figure 3 a shows the rib pattern after the resist development while the SEM images of Figs. 3b and c show that the rib waveguide is 400 nm wide and has smooth and perpendicular walls, resulting from the successful anisotropic etch. The slab height of the SOI wafer was measured with the profilometer and resulted in 220 nm (Fig. 3d), confirming that Si was etched down to the buffer oxide. Since charging occurs (due to oxide) while taking the SEM image, the best image was tried to be taken by carbon coating. A few nm of carbon coating was erased again with oxygen plasma.

After the SOI waveguide was created, a 5 nm thick aluminum oxide (Al₂O₃) layer was deposited on top of the waveguide by ALD (Okyay Tech ALD Atomry T8) at 200 °C which is a low substrate temperature. Trimethylaluminum [TMA, Al (CH₃)₃] was used as a precursor material of Al₂O₃. The growth per cycle (GPC) was 1.28 A/cycle.

For reflectance/transmittance measurements, the Bentham PVE300 photovoltaic characterization system was used to determine the spectral responsivity, reflectance, and transmittance of the device. A dual Xenon/quartz halogen light source with a wavelength of 300–1800 nm was coupled to a monochromator.

After the transfer of Gr, in the final step, 50 nm thick Al metal (purity 99.99%) and 50 nm thick Ag metal (purity 99.99%) were used for ohmic contacts to graphene and silicon, respectively. The contacts were deposited by Thermal Evaporation (Bestec Thermal Evaporation System) technique by a 1.5 × 1.5 mm² metal mask. The contacts were annealed in a vacuum under 4.1 × 10⁻⁵ mbar pressure at 350 °C for 3 min.