Nanometer thin-film Ni-NiO-Ni diodes for detection and mixing of 30 THz radiation

Abstract

We report on the realization and the experimental study of thin-film Ni-NiO-Ni diodes with integrated infrared antennas. These diodes are applied as detectors and mixers of 28-THz CO2-laser radiation with difference frequencies up to 176 GHz. They constitute a mechanically stable alternative to the point-contact MOM diodes used today in heterodyne detection of such high frequencies. Thus, they represent the extension of present millimeter-wave and microwave thin-film and antenna techniques to the infrared. Our thin-film Ni-NiO-Ni diodes are fabricated on SiO2/Si substrates with the help of electron-beam lithography at the IBM Research Laboratory (Rüschlikon, Switzerland). We have reduced the contact area to 110 nm × 110 nm in order to achieve a fast response of the device. This contact area is in the order of those of point-contact diodes and represents the smallest ever reported for thin-film MOM diodes. The thin NiO layer with a thickness of about 35 Å is deposited by sputtering. Our thin-film diodes are integrated with planar dipole, bow-tie and spiral antennas that couples the incident field to the contact. The second derivative I″(V) of the nonlinear I(V) characteristics at the bias voltage applied to the diode is measured at a frequency of 10 kHz. It determines the detection and second-order mixing performed with the diode for frequencies from dc to at least 30 THz. The I″(V) characteristics exhibit for low bias voltage Vbias a linear dependence, which is followed by a saturation and a maximum for high Vbias. The zero-bias resistance of the diode is in the order of 100 Ω. It is not strictly inversely proportional to the contact area of the diode. The first application of our thin-film diodes was the detection of cw CO2-laser radiation. The measured dc signal generated by the diode when illuminated with 10.6-μm radiation includes a polarization-independent contribution, caused by thermal effects. This contribution is independent of the contact area and of the type of integrated antenna. The polarization-dependent contribution of the signal originates in the rectification of the antenna currents in the diode by nonlinear tunneling through the thin NiO layer. It follows a cosine-squared dependence on the angle of orientation of the linear polarization, as expected from antenna theory. For the linearly polarized dipole and bow-tie antennas, the maximum detection signals are therefore measured for the polarization parallel to the antenna axis. Bow-tie antennas with a half length of 2.3 μm generate the highest detection signals. The full length of these antennas corresponds to 3/2 of the wavelength of the incident 10.6-μm radiation in the supporting Si substrate. The relevance of the substrate wavelength confirms that our antennas are more sensitive to the radiation incident from the substrate side. The time of response of our thin-film diode is not limited by the speed of the electron-tunneling effect, but by the RC time constant of the diode circuitry. Thus, the overall best performances are attained by the diodes with the smallest contact areas and corresponding capacitances. The study of the polarization response of our integrated asymmetric spiral antennas revealed the contribution of an unbalanced mode propagating on the antenna arms beside the fundamental balanced mode. The imbalance is caused by the reactive impedance of the diode and by the asymmetry of the antenna arms in the feed region. In addition, the response of the diode is influenced by reflection of the antenna currents near the end of the spiral arms. The resulting polarization of our spiral antenna is therefore not the expected circular polarization, yet an elliptical polarization with an axial ratio in the order of 0.12. Furthermore, we have demonstrated the presence of two distinct additive thermal effects besides the fast antenna-induced contribution by the measurement of the response of our thin-film diodes to 35 ps optical-free-induction decay (OFID) CO2-laser pulses. The measured characteristic times of these two relatively slow relaxations are τ1 ≈ 100 ns and τ2 ≈ 15 ns. These exponential relaxations observed are explained by thermal diffusion in the SiO2 and in the Ni layers of our structures. These time constants show that thermal effects influence mixing processes at low difference frequencies. For the first time, the operation of thin-film diodes as mixers of 28-THz radiation was demonstrated. Difference frequencies up to 176 GHz have been measured when the diode was irradiated by two CO2-laser beams and microwaves generated by a Gunn oscillator working at 58.8 GHz. These difference frequencies were generated in mixing processes from the second to the fifth order. These experiments were performed with thin-film Ni-NiO-Ni diodes with the minimum contact area of 0.012 μm2 and integrated resonant bow-tie antennas. The transmission of the high-frequency signals to the spectrum analyzer was accomplished using integrated rhodium waveguides and flip-chip connections. The diode and the antenna were irradiated through the substrate, taking advantage of the better sensitivity of the antenna to radiation incident from the substrate side. The dependence on the linear polarization of the mixing signal matches almost perfectly the ideal cosine-squared dependence predicted by antenna theory for bow-tie antennas. A ratio of the mixing signals for the polarization parallel to the axis vs. the cross-polarization of over 50 was attained. The signal-to-noise ratios of our mixing signals demonstrate the potential of our type of diodes to respond to even higher carrier and difference frequencies. Also higher-order mixing can be achieved with our thin-film diodes. © 1998 Elsevier Science B.V. All rights reserved.