Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna

A critical challenge for the convergence of optics and electronics is that the micrometre scale of optics is significantly larger than the nanometre scale of modern electronic devices. In the conversion from photons to electrons by photodetectors, this size incompatibility often leads to substantial penalties in power dissipation, area, latency and noise. A photodetector can be made smaller by using a subwavelength active region; however, this can result in very low responsivity because of the diffraction limit of the light. Here we exploit the idea of a half-wave Hertz dipole antenna (length 380 nm) from radio waves, but at near-infrared wavelengths (length 1.3 mm), to concentrate radiation into a nanometre-scale germanium photodetector. This gives a polarization contrast of a factor of 20 in the resulting photocurrent in the subwavelength germanium element, which has an active volume of 0.00072 mm, a size that is two orders of magnitude smaller than previously demonstrated detectors at such wavelengths. The interaction of light with nanostructured metals has been studied extensively in recent years. The resulting near-field optical intensity can be two to three orders of magnitude higher than the incident intensity. However, very little research has been carried out into the interaction of these strong near fields with semiconductors and the further transformation of the optical energy into electricity. It has recently been demonstrated that the photogeneration of carriers in silicon can be enhanced by a surface-plasmon antenna at a wavelength of 840 nm (ref. 12). This method has the practical limitation that the entire grating structure necessary for exciting a surface-plasmon resonance occupies a large area in terms of wavelengths. Alternatively, a C-shaped aperture has been used to enhance photodetection locally without exciting long-range surface-plasmon resonances. However, for easy integration and high-speed, low-capacitance operation, it is generally advantageous to design planar devices such as the metal–semiconductor–metal (MSM) detectors that are widely used in high-speed optical receivers. Resonant antennas can confine strong optical near fields in a subwavelength volume, as demonstrated recently for bow-tie antennas and dipole antennas at visible wavelengths using the resulting scattered light. The optical properties of the structures largely depend on the size and shape of the antennas. Using the principle of high field enhancement by an antenna, we present a deeply subwavelength MSM photodetector. Figure 1 shows a schematic of the device structure. The open-sleeve dipole antenna made of gold consists of a dipole oriented along the y direction and two line electrodes (sleeves) along the x direction. A germanium nanowire lies under the two line electrodes and in the gap region between the two dipole arms. Open-sleeve dipole antennas were initially proposed for radio frequencies to increase the bandwidth of an ordinary dipole antenna. In our device, the dipole was used to collect light from a large area and concentrate it into the small subwavelength region of the germanium. The sleeves were used to extract photocurrent without substantially changing the antenna characteristics (from a bare dipole). Crystalline germanium was chosen to be the active material of our photodetector because of its high responsivity at near-infrared wavelengths and its compatibility with standard silicon technology. Previous research has shown that use of a substrate with a high dielectric constant significantly z y Oxide