NANOMETER THIN-FILM NI-NIO-NI DIODES FOR DETECTION AND MIXING OF 30 THZ RADIATION

We report on the realization and the experimental study of thin-film N i-NiO-Ni diodes with integrated infrared antennas. These diodes are ap plied as detectors and mixers of 28-THz CO2-laser radiation with diffe rence 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 techni ques 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 (Ruschlikon, Switzerland). We have reduced the contact area to 110 nm x 110 nm in order to achieve a fast response o f the device. This contact area is in the order of those of point-cont act diodes and represents the smallest ever reported for thin-film MOM diodes. The thin NiO layer with a thickness of about 35 Angstrom is d eposited by sputtering. Our thin-film diodes are integrated with plana r dipole, bow-tie and spiral antennas that couples the incident field to the contact. The second derivative I ''(V) of the nonlinear I(V) ch aracteristics at the bias voltage applied to the diode is measured at a frequency of 10 kHz. It determines the detection and second-order mi xing performed with the diode for frequencies from de to at least 30 T Hz. The I ''(V) characteristics exhibit for low bias voltage V-bias a linear dependence, which is followed by a saturation and a maximum for high V-bias. The zero-bias resistance of the diode is in the order of 100 Omega. It is not strictly inversely proportional to the contact a rea of the diode. The first application of our thin-film diodes was th e detection of cw CO2-laser radiation. The measured de signal generate d by the diode when illuminated with 10.6-mu m radiation includes a po larization-independent contribution, caused by thermal effects. This c ontribution is independent of the contact area and of the type of inte grated antenna. The polarization-dependent contribution of the signal originates in the rectification of the antenna currents in the diode b y nonlinear tunneling through the thin NiO layer. It follows a cosine- squared dependence on the angle of orientation of the linear polarizat ion, as expected from antenna theory. For the linearly polarized dipol e and bow-tie antennas, the maximum detection signals are therefore me asured for the polarization parallel to the antenna axis. Bow-tie ante nnas with a half length of 2.3 mu m generate the highest detection sig nals. The full length of these antennas corresponds to 3/2 of the wave length of the incident 10.6-mu m radiation in the supporting Si substr ate. The relevance of the substrate wavelength confirms that our anten nas are more sensitive to the radiation incident from the substrate si de, 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 capac itances. The study of the polarization response of our integrated asym metric 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, th e response of the diode is influenced by reflection of the antenna cur rents near the end of the spiral arms. The resulting polarization of o ur 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 add itive thermal effects besides the fast antenna-induced contribution by the measurement of the response of our thin-film diodes to 35 ps opti cal-free-induction decay (OFID) CO2-laser pulses. The measured charact eristic times of these two relatively slow relaxations are tau(1) appr oximate to 100 ns and tau(2) approximate to 15 ns. These exponential r elaxations observed are explained by thermal diffusion in the SiO2 and in the Ni layers of our structures. These time constants show that th ermal effects influence mixing processes at low difference frequencies . For the first time, the operation of thin-film diodes as mixers of 2 8-THz radiation was demonstrated. Difference frequencies up to 176 GHz have been measured when the diode was irradiated by two CO2-laser bea ms and microwaves generated by a Gunn oscillator working at 58.8 GHz. These difference frequencies were generated in mixing processes from t he second to the fifth order. These experiments were performed with th in-film Ni-NiO-Ni diodes with the minimum contact area of 0.012 mu m(2 ) and integrated resonant bow-tie antennas. The transmission of the hi gh-frequency signals to the spectrum analyzer was accomplished using i ntegrated 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 mixin g signal matches almost perfectly the ideal cosine-squared dependence predicted by antenna theory for bow-tie antennas. A ratio of the mixin g signals for the polarization parallel to the axis vs. the cross-pola rization of over 50 was attained. The signal-to-noise ratios of our mi xing signals demonstrate the potential of our type of diodes to respon d to even higher carrier and difference frequencies. Also higher-order mixing can be achieved with our thin-film diodes. (C) 1998 Elsevier S cience B.V. All rights reserved.