Cornell NanoScale Science & Technology Facility

Electronic generation of THz signals using GaN is predicted to exhibit high efficiency. Theory shows that electrons can be ballistically accelerated to the Brillouin zone energy limit of 2.7 V [1,2], and above 1.0 V, they will pass beyond the inflection point and have negative effective mass. Experiments by M. Wraback [3] indicate the presence of negative differential resistance (NDR). Finite energy injection at 0.4 to 0.6 eV launches electrons closer to the inflection point to increase the ratio of negative to positive mass electrons. Summary: First generation devices were n+-i-n+ type structures (2 x 1019/cm3 n+ doping) with high contact and spreading resistances that prevented establishment of the necessary amount of voltage drop across the intrinsic layer (ilayer) to push electrons beyond the inflection point. The domination of the parasitic resistances in the device behavior is seen for the I-V comparison of an actual device with a planar no-mesa diagnostic version. Almost all of the bias voltage drops across the parasitics and only a small portion of it falls across the i-layer. Moreover, bias voltages above approximately 3 volts result in heating at the contact to a high enough temperature to vaporize the connection at the air bridge. The high contact resistances to n+ regions were unexpected because the same ohmic contact technology used for AlGaN/GaN FET’s was employed. A series of TLM experiments with various metals were performed and as-deposited Ti/Al/Mo/Au proved to be the best with a specific contact resistance of 4.92 x 10-8 ohm-cm2 for 1.26 x 1020/cm3 n+ doping. This showed a hundred to one reduction in the contact resistance. Second generation devices included an AlGaN launcher to increase injection energy. The design utilized non-alloyed Ti/Al/Mo/Au ohmic contacts on the AlGaN/GaN heterostructures. Long periods of operation at high bias resulted in change in parasitic resistance. While bottom ohmic contacts exhibited low resistance, the hotter top contact was found to be the source of the unstable high resistance. It is not conclusive how much role the unstable, high resistance anode ohmic contact plays in the observation of the negative differential resistance. We usually process 15 x 15 mm square samples cut out from a 2 inch GaN wafer. GaN is grown on sapphire substrates using our own MBE facility in Phillips Hall. The epilayer consists of doped and undoped GaN and AlGaN layers. Thus far we’ve tried both n+-i-n+ structures and AlGaN/GaN heterostructures, both having an ilayer thickness ranging from 600Å to 300Å. Fabricated devices have a circular active region, defined by a dry etch process (chlorine based ICP). The diameters of these devices are 3, 5, 10, 15 and 25 μm. Then, two different interconnect types were fabricated after the deposition of the ohmic contacts. First type required the deposition of a 2 μm thick silicon dioxide layer to provide the dielectric medium for the fabrication of on-chip microwave resonators. These were attached to some of the devices to force them to oscillate at certain frequencies in case NDR is present. Sloped holes in the dielectric were then drilled to make electrical contact to the devices through appropriate size cascade probe pads. The other type employed a standard airbridge process connecting the two terminals of the devices directly to these pads.